Principal Why Are Orangutans Orange?: Science Questions in Pictures - With Fascinating Answers: More Questions..

Why Are Orangutans Orange?: Science Questions in Pictures - With Fascinating Answers: More Questions and Answers from the Popular ’Last Word’ Column

Illustrated for the first time, with eighty full-colour photographs showing the beauty, complexity and mystery of the world around us, here is the next eagerly awaited volume of science questions and answers from New Scientist magazine. From ripples in glass to OCyholograms' in ice, the natural world's wonders are unravelled by the magazine's knowledgeable readers. Six years on from Does Anything Eat Wasps? (2005), the New Scientist series still rides high in the bestseller lists, with well over two million copies sold. Popular science has never been more absorbing or more enjoyable. Like Why Don't Penguins' Feet Freeze? (2006), Do Polar Bears Get Lonely? (2008) and Why Can't Elephants Jump? (2010), this latest collection of resourceful, wry and well-informed answers to a remarkable range of baffling science questions is guaranteed to impress and delight."
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Páginas: 224
ISBN 10: 1847657559
ISBN 13: 9781847657558
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Why Are Orangutans Orange?

Science questions in pictures –

with fascinating answers

Why Are Orangutans Orange?

Science questions in pictures –

with fascinating answers

More questions and answers

from the popular ‘Last Word’ column

edited by Mick O’Hare

First published in Great Britain in 2011 by

Profile Books Ltd

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Copyright © New Scientist 2011

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ISBN 978 1 84668 507 1

eISBN 978 1 84765 755 8

Text design by Sue Lamble

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Printed in China through Asia Pacific Offset Ltd



1 All creatures great and peculiar

2 Ice, bubbles and liquid

3 Clouds and stuff in the sky

4 In your kitchen

5 Goo and gardening

6 Bugs and blobs from the deep

7 Sand, saws and the Klingons




OK, we admit it, they’re cute. That’s why they are on the front of the book. But when asked, we had no idea why orangutans were a strange orange colour – one that didn’t even seem to match their environment. It was a long time before we received any response to the question too, suggesting that even the experts were a little unsure. But now we think we know – turn to page 30 to find out.

You’ll also discover among these pages why tigers have stripes rather than spots, why blue-footed boobies have, erm, blue feet and whether kittiwakes can fly upside down. And it’s not all about animals. We have the lowdown on any number of clouds, strange bubbles and weird ice… and all in glorious colour.

Readers of our earlier books such as Why Don’t Penguins’ Feet Freeze? and Does Anything Eat Wasps? will notice a difference in this latest collection of questions and answers from New Scientist’s Last Word column – photographs: and lots of them.

Most haven’t been supplied by professionals. In fact, nearly all have been taken by readers of The Last Word column in New Scientist magazine and on its website. Some of the photographs are extraordinary, many are unique and some are a bit fuzzy. But we can handle that because they tell the visual story of some extraordinary phenomena, taken on the run by members of the public.

And that is the essence of this book: a celebration of the wonder of our world that any inquisitive person lucky enough to be in the right place at the right time can witness and record – if we are prepared to keep our eyes open. Specialists in their fields have spent years waiting to capture these moments but they have been beaten to it by readers of New Scientist and its books.

All of which means we can now tell you the story of why flies sometimes explode, what you should do when your hair stands on end (and why it’s very, very important to do it quickly) and why Mount Fuji sometimes appears to be wearing a hat.

If you have any similar images that you have captured somewhere in the world – from your back garden to coldest Antarctica – and have always wondered what on earth they show, The Last Word can help. Every week hundreds of questions pour into our offices, some with photographs, others without. You can add yours to the list, or help us answer the ones we are still puzzling over. Visit to ask a question, or help us answer one. And buy the magazine to check out our weekly page. You could even appear in the next book (or at least your photograph could).

Mick O’Hare

1 All creatures great and peculiar

Happy feet

The blue-footed booby is an extraordinary-looking bird. It has fairly dull plumage but strikingly coloured blue legs and feet. What could be the evolutionary benefit of such a conspicuous feature? Both sexes have blue feet so they don’t seem to be for impressing potential mates.

Sam Moore

London, UK

Although not obvious at first sight, during courtship blue-footed boobies (Sula nebouxii) have different-coloured feet depending on their sex: male feet are brighter and more of a greenish-blue, while the females have duller feet that are bluish.

The birds exhibit their feet to prospective partners in a series of courtship displays. These include a kind of ritualised strutting around that allows them to show off their feet, plus stylised or ‘salute’ landings which serve the same purpose.

I am a member of a research group that studies the sexual behaviour of the blue-footed booby. In one experiment, we altered the colour of the courting males’ feet and recorded the females’ response. Females paired to males with duller feet were less enthusiastic about courtship and less likely to copulate compared with females paired to males with normal, brightly coloured feet. Similarly, when we altered the females’ feet to a duller blue, males became less interested in courting them. Birds in poor health often have dull blue feet.

What’s more, females whose mates had dull blue feet produced smaller eggs, and their chicks had a poorer immune response compared with normal females. This may sound surprising, but it is in accordance with theoretical expectations.

All this suggests that males are probably under strong selection pressure to maintain greenish-blue feet during courtship. This will ensure not only that they copulate successfully but also that their mates will lay big, healthy eggs. Overall, our results suggest that foot colour is a trait maintained by mutual male and female preferences.

Roxana Torres

Institute of Ecology

National Autonomous University of Mexico

Both male and female Sula nebouxii have blue feet, but it is the male that presents his feet prominently in courtship. This, in effect, is a way of saying that he is of the same species as the female.

I cannot offer any specific reason why the blue-footed booby has blue feet, but I would point out that foot colour does seem to be significant in the genus – there is an equally striking red-footed booby, Sula sula. This suggests that as members of the genus evolved, they adapted to different ecological niches which, in turn, meant that there was an advantage in the birds splitting into different ‘tribes’ that could only mate with their own kind.

This is an example of what is called sympatric evolution, where one species evolves into two within a shared territory. In contrast, allopatric evolution occurs because populations become isolated from each other. For sympatric evolution to succeed, it is essential that some sort of difference between the species arises so that a bird can distinguish between a bird of a related species and one of its own kind.

Guy Cox

Associate Professor

Australian Centre for Microscopy & Microanalysis

University of Sydney, Australia

Ducking the issue

I have never seen a duck stand as erect as the one shown in the centre of this photo, which I took at Rowsley, Derbyshire. Does anyone know if there is an explanation for this posture or is it just an unexpectedly tall duck?

Vince Sellars

Sheffield, UK

Most of the birds in the background of the photograph are male and female mallards (Anas platyrhynchos) from which almost all domestic ducks originate and hence commonly and freely interbreed.

But the upright drake is a cross-breed – note the less-clearly defined markings compared with the other drakes. He is half mallard and his other parent was an Indian runner. This is a common breed that is raised for its egg-laying performance and is characterised by its distinctive vertical stance and slender frame, which results in a comical gait. Standard domestic ducks similar to the others in the photograph, which are bred for their meat, retain a more normal horizontal carriage.

Interestingly, the slender upright stance seen in this duck is quite dominant genetically, and interbreeding between Indian runners and other ducks typically results in skinny, upright offspring. Indian runners come in a wide variety of colours, with white and brown being the most common.

Giles Osborne

Mitcham, Surrey, UK

The erect duck is a hybrid of a mallard duck and a domestic Indian runner duck. Indian runners and the crested version, Bali ducks, came from Indonesia – not India – and were brought to Europe by Dutch traders. They were once known as penguin ducks because of their erect stance.

Gail Harland

Coddenham Green, Suffolk, UK

For those who would like to explore the parentage and history of this bird further, check out the Indian Runner Duck Association at Thanks to Jo Horsley of Llanwrda, Carmarthenshire, UK, and others for pointing this out – Ed.


With the exception of the sperm whale’s off-centre blow-hole and some crabs’ single large claw, all complex organisms I can think of are effectively symmetrical along one plane of their body. What is the least symmetrical organism?

Max Maguire

By email; no postal address supplied

Flatfish received the largest vote, but there are plenty of other strange candidates out there – Ed.

The least symmetrical organism is the halibut, which has both eyes on the same side of its head.

Donald Windsor

Norwich, New York, US

There are different symmetries in nature. We tend to assume bilateral symmetry is normal because that is what we and most of the organisms we notice (vertebrates and arthropods) display. But bilateral symmetry is the exception rather than the rule; many creatures exhibit radial or even spherical symmetry. Some alter their symmetry over time – for example, a starfish will start out as a bilaterally symmetrical larva and become radially symmetrical as it matures. Humans, a few days after conception, are basically a spherically symmetrical organism called a morula.

Many organisms do not have any clear geometrical symmetry but demonstrate some kind of fractal symmetry, where their structures look similar at a variety of scales. Many plants and fungi are a bit of both: think of the leaves and the apples on an apple tree.

Humans are not quite bilaterally symmetrical: our liver is on the right side, our spleen on the left, while our right lung has three lobes and the left two. We even slip into fractal symmetry when it suits the purpose: take a close look at the capillaries which transport blood to the tissues. We are not even superficially symmetrical. Next time you get out of the bath ask yourself, ‘Do they both hang the same?’ This works for either sex.

We are all changed and shaped by both our genes and our environment. To put it another way, we all conform to a pattern while being eccentric. Heck, that’s life.

David Hopkins

Smethwick, West Midlands, UK

Members of the genus Histioteuthis, squid that live down to depths of 1000 metres, are unique in the animal kingdom as their left eye is two to three times the size of the right. The reasons for this trait, which gives rise to its common name of the cock-eyed squid, are unclear. There is also a corresponding asymmetry in the optic lobes of the squid’s brain. The specimen pictured below was filmed on board ship after being caught off the coast of California.

Ron Douglas

Department of Optometry & Visual Science

City University, London, UK

One suggestion is that the depth at which cock-eyed squid live is about as far down as sunlight can penetrate. The squid trains one eye on the illuminated water above while the other looks down into blackness – Ed.

Asymmetry is commonest among organisms that have little need of well-defined structures in their bodies. Some algae, fungi and sponges never developed much symmetry, while parasites can abandon symmetry when they grow opportunistically to secure food. An example of the latter is Sacculina, a barnacle that injects its soft body through a crab’s shell and then grows a lump of reproductive tissue plus a tangle of feeding filaments throughout the crab’s body. And some members of the group of tiny crustaceans known as copepods form shapeless reproductive sacs within cysts in the flesh of fishes. Such creatures need no symmetry.

Jon Richfield

Somerset West, South Africa

There is a wonderfully quirky group of asymmetric barnacles called verrucomorphs, shown in the photograph opposite. They are either ‘right-handed’ or ‘left-handed’ – apparently a random choice – their form being determined by the loss of calcareous plates from either the right or left side of the shell wall, plus a reduction in the number of plates in the shell’s lid to two from the usual four. Why they have adopted this asymmetric form is uncertain, particularly as both forms occur together. Their soft tissue, as it happens, retains bilateral symmetry. These and other asymmetric barnacles were first comprehensively described by Charles Darwin in a monograph published in 1854.

John Buckeridge

RMIT University

Melbourne, Australia


One of my young chickens has just produced an unusually coloured egg (at the right of the photo). The egg on the left is more typical of the breed. I know egg shell colour is variable, even in eggs laid by the same hen on different days, but how did one egg undergo such a sudden and distinct colour change?

Colin Booth

Durham, UK

The answer probably lies in the fact that, until shortly before they are laid, hens’ eggs are white. The brown pigmentation associated with breeds such as the Rhode Island red and the maran is a last-minute addition during egg formation and, like a fresh coat of paint, can come off surprisingly easily.

More than 90 per cent of the shell of a hen’s egg consists of calcium carbonate crystals bound in a protein matrix. The shell starts to form after the egg has reached the uterus, where it stays for around 20 hours prior to being laid.

During this time, glands secrete the shell around the membranes that hold the yolk and albumen. In brown-egg-laying breeds, the cells lining the shell glands release pigment during the last 3 to 4 hours of shell formation. Most of the pigment is transferred to the cuticle, a waterproof membrane that surrounds the porous eggshell.

Several factors can disturb the cuticle formation process and thus pigmentation, such as ageing, viral infections – including that perennial chicken farmer’s nemesis, bronchitis – and drugs such as nicarbazin, which has been widely fed to poultry to combat a disease caused by a type of protozoan. Possibly the most significant factor affecting egg pigmentation is exposure to stress during the formation of the egg.

If a flock of hens is disturbed by a fox during the night, for example, they might well lay paler eggs in the morning. The adrenaline the hens release puts egg-laying on hold and shuts down shell formation. The egg’s pigmentation will be affected if the cuticle doesn’t form properly.

Even if the pigment is laid down, there is no guarantee that it will last, as Morris Steggerda and Willard F. Hollander found in 1944 while they were studying eggs from a flock of Rhode Island reds in the US. When they cleaned the eggs, the brown pigment occasionally came away; the harder the eggs were rubbed, the more pigment was removed. Only those shells with a glossy sheen retained their colour, suggesting their cuticles had been fully formed, with a protective layer that acted rather like the varnish on an oil painting.

As for the egg photographed by your questioner, the bird was probably disturbed while the cuticle was being formed and so the pigment, inadequately protected, was rubbed off the larger, rounder end of the egg as it was forced out.

The issue may have some significance for public health. The waterproof cuticle is the egg’s defence against bacteria. As shell colour is affected by how well the cuticle forms, it also provides a visual test of how free from harmful bacteria an egg may be.

Hadrian Jeffs

Norwich, Norfolk, UK

Before an egg is laid, the hen’s shell gland secretes pigment into the fluid bathing the egg’s surface. The fluid smears readily, and any disturbance while the egg dries can create marks. Farmers are therefore fussy about the kind of bedding they use in nesting boxes.

Eggs are usually laid big end first. The hen that laid the egg in question may have resorted to using friction to release the partly laid egg from its cloaca, possibly by rubbing the egg against the bedding it was sitting on.

Alternatively, the hen may have paused halfway through laying, perhaps disturbed or exhausted, with the egg half-protruding from its cloaca. The part of the egg still within the cloaca had time to achieve a deep colour before the hen relaxed again, and this accounts for the sharp boundary in coloration seen in the photo. Such a scenario is unusual but it does happen.

Jon Richfield

Somerset West, South Africa

Life on Uluru

Some decades ago I was lucky enough to climb Uluru in Australia’s Northern Territory. Recent rain had left pools on top of the rock and, curiously, in many of them there were strange aquatic invertebrates as seen in the photo. They look like ancient trilobites. Why and how are they on top of the famous, massive rock, and what are they? What happens to the creatures when the puddles dry up?

Gavin Chester

Dwellingup, Western Australia

The animals pictured are shield shrimps, Triops australiensis. They are crustaceans in the class Branchiopoda – meaning ‘gill-legged’ – and this term reflects the fact that they use their legs for breathing as well as for movement.

Their external morphology appears to have remained unchanged for 220 million years or more, and one shield shrimp, Triops cancriformis, has been claimed by some to be the oldest extant animal species. They occur in bodies of fresh or slightly salty water that periodically dry out, such as ephemeral lakes, farm dams, ditches and even puddles left after rain.

The eggs of these animals have a very strong shell and are resistant to drying out. In some species, a period of desiccation is necessary for the creature’s development. The eggs can tolerate freezing and temperatures up to 80 °C, and may remain viable for 25 years. In some species, hatching may take up to a year following exposure to suitable conditions, but in T. australiensis it usually takes several weeks at most. Once hatched, development from egg to adult may take only a further few weeks in summer temperatures. The animals have a lifespan of up to three months, and adults reach a length of about 35 millimetres.

The shrimps feed on microscopic organisms, aquatic worms, other shrimp species, frogs’ eggs and tadpoles, decomposing vegetation and other detritus, and sometimes even moulting individuals of their own species. The small size and the robustness of the eggs allow them to be carried on the wind for hundreds of kilometres from their pools of origin, and it is probably this mode of transport that would have delivered the eggs to the top of Uluru.

It is also possible that the eggs might have been carried up in mud caked on a visitor’s boots. Although in this instance such a method of transport is essentially innocuous, it is nevertheless a salient reminder of the need to ensure that all clothing and equipment is cleaned before moving from one ecosystem to another.

Harko Werkman

Woodbridge, Tasmania, Australia

I bought a packet of desiccated shield shrimp eggs (Triops australiensis) on the internet for my boyfriend’s 30th birthday. As the name Triops suggests, shield shrimps have three eyes: two compound eyes and one ‘naupliar eye’ – a simple median eye, first appearing in the larval stage. They closely resemble their Triassic ancestors, which existed around 220 million years ago.

Blown around with the dust, eggs eventually settle in crevices and grooves – even on the top of the great rock – where they may remain viable for up to 10 years. I guess that means my boyfriend has an excuse for not hatching them yet.

Kate Hutson

School of Earth and Environmental Sciences

University of Adelaide, Australia

Triops species are found on most continents but are rare in the UK, where the tadpole shrimp (Triops cancriformis) is currently known to exist in only two locations, the New Forest in southern England and the Solway Firth in southwest Scotland. Visitors to the Wildfowl and Wetlands Trust Centre at Caerlaverock, Dumfriesshire, can view this species in the visitor centre.

Triops is typical of ephemeral, or temporary, wetlands, and can survive drying to persist for up to 30 years as eggs or cysts. The eggs at Caerlaverock were collected to provide a safety net for the population in Scotland, where they persist in one temporary pond which has been created by cattle trampling around a fence post.

Emma Hutchins and Sally Cordwell

Wildfowl and Wetlands Trust

Slimbridge, Gloucestershire, UK

Bird on a wire

I saw this kittiwake flying upside down in Norway’s Svalbard archipelago – about 79° North – while I was stuck in the sea ice. This and other kittiwakes were feeding on polar cod (about 13 centimetres long) that had become uncovered as our ship broke through the ice. What is it doing and why? How many other birds can do this?

Bill Reed


The kittiwake (Rissa tridactyla) is not flying upside down at all. You can tell because the bird’s upper wings are visible in the photo, showing the silvery-grey feathers with the classic dipped-in-black-ink wing tips. If it were upside down we would see the underwings, which are white with black tips. The twist of the bird’s head is interesting, though. Clearly the bird has turned its head a long way to the right, so that it seems initially to be flying upside down.

Many birds can rotate their heads to this degree or more – owls and other birds of prey are the best-known examples. In these species, head-turning helps them to detect their prey. Specifically, it allows owls to orientate their ears to obtain the best possible reception when listening for the faint rustling of a rodent moving through vegetation in the dark.

In the kittiwake, however, this doesn’t happen. A possible explanation is that the bird is trying to cough up a particularly sharp piece of fish bone or something else it has swallowed. Many birds, including gulls, regurgitate indigestible pellets.

Another explanation is that the bird is shaking off excess salt water from its beak. Most seabirds take in varying amounts of salt water when feeding, which they have to get rid of before it reaches harmful levels in the body. Finally, the bird may simply be twisting as it calls out to other individuals in the same area, or just keeping a lookout for potential predators, such as skuas.

Kevin Elsby

Norwich, Norfolk, UK

If you get a chance to look at the photos in the websites below (especially the first) do so – Ed.

The bird is flying normally and twisting its head around, perhaps to preen itself or to loosen a morsel of fish that it may be eating. The underwing pattern of the kittiwake looks nothing like the upper wing at all, and a cursory inspection of the structure of the flight feathers of this bird reveals a normally aligned gull.

Birds do not generally fly upside down, but they may momentarily invert, such as when wildfowl ‘whiffle’ to lose height rapidly, spilling air from under their wings (see bit. ly/3wfoqV). Additionally, some birds may roll during mating displays, such as the aptly named roller birds, part of the order Coraciiformes, while others might in play (see

Simon Woolley

Winchester, Hampshire, UK

All alone

Do polar bears get lonely? I’m not being flippant, just attempting to find out why animals such as humans or penguins are gregarious while others, such as polar bears and eagles, live more solitary lives.

Frank Anders

Amsterdam, The Netherlands

Readers will recognise this question as the title of one of our earlier books, Do Polar Bears Get Lonely? While we are not going to repeat all the answers here – you’ll have to buy the book if you haven’t already – we were impressed with this photograph of a polar bear and a husky, taken by Norbert Rosing. They are seemingly getting along just fine. If polar bears do indeed get lonely, every so often, it seems, they are prepared to cavort with their fellow animals in the Arctic – Ed.

Batty behaviour

About a million bats fly out of the Mulu Cave in Sarawak, Malaysia, every evening to look for food. They fly out in batches and it sometimes takes two hours before they are all in the air. When they leave the cave they form a circle before forming sinusoidal waves that stretch great distances. Why do they fly like this?

Mazlan Othman

Putrajaya, Malaysia

The sinusoidal wave still seems to be a matter of conjecture – Ed

Mexican free-tailed bats emerging from caves in the southern US fly in the same circular pattern, and around the British Isles puffins approaching their burrows also fly in a ‘wheel flock’. Fast-flying bats or birds adopt this formation as a protection against predators such as hawks, gulls or skuas.

I am unsure about the sinusoidal movement. Perhaps it arises because bats are safer from predators near the ground but have to seek their food higher in the sky. This could mean that sections of the flock briefly climb high in food-gathering sorties – while it is still light enough for the hawks to see them – before ducking back to low altitude for safety. Mexican free-tailed bats, for example, feed very high in the air after darkness has descended.

Sean Neill

University of Warwick

Coventry, UK

Bat sonar is oriented directly ahead of the bat and would be blocked if they flew in a straight line. By flying in a spiral each can maintain a forward view and use its sonar to detect prey or predators. Seen from behind (as in the photo) the spiral appears as a circle and from the side as a sinusoidal wave. It is remarkable how perfect the spiral is and it would be interesting to know if it is always clockwise or anticlockwise for a given species.

Jerry Whitman

Barnham, West Sussex, UK

Froggy fear

I was spring-cleaning my pond and was horrified to discover what I thought was a dead fish. On closer inspection, the fish, a golden orfe, was still alive but had a frog firmly attached to it. The frog was clasping the fish as though it was trying to mate with it. I have never seen anything like it. I also noticed that several of my other normally healthy fish seem to have sustained injuries. Is a killer frog on the loose?

Clare Dyer

London, UK

This is a case of misdirected sex drive and a lonely, rather than a killer, frog. Most frogs and toads mate in the water by means of a process called amplexus, in which the male clings to the female’s back and releases his semen into the water as the eggs are laid so that fertilisation occurs externally. Males are equipped with rough ‘nuptial pads’ on their forelimbs which help them to grip onto the female during this process.

The strength of the male’s urge to mate is dramatically demonstrated by the common toad, Bufo bufo, which can often be seen forming large clumps in which a single female is tightly surrounded by a large number of males. Where the sex ratio is particularly unfavourable, males will cling onto a variety of inappropriate objects, both animate and inanimate, in a futile attempt to mate.

Jonathan Wallace

Newcastle upon Tyne, UK

When the pond is overstocked with male frogs and there are not enough females to go round, the desperate males will attach to anything. If the fish is small enough it will die because the frog will cover its gills with its forelegs, thus suffocating the poor creature. I have witnessed this several times. The answer is to remove all large male frogs from your pond in early spring.

Dave Gaskell

Tranmere, Merseyside, UK

I feel compelled to suggest that perhaps what your correspondent observed is not a frog at all, but a horny toad.

Sorry, I couldn’t resist.

Ben Haller

Menlo Park, California, US

Shell shock

Dining out in Belgium, some of our party ate snails in garlic, and one took home an empty shell for his 3-year-old son to play with. The washed shell sat on the kitchen work surface most of the time, until one day two baby snails emerged from it. The ‘parent snail’ had long since been fried, scooped out and eaten. Assuming my friend is not hoaxing us, what happened here?

Dave Mitchell

By email; no postal address supplied

Several readers thought this question was indeed an elaborate hoax. But there may be a simple answer – Ed

Snails both fertilise and carry their eggs internally. When being prepared for the table, the snails are scooped out of their shells, usually mixed with butter, parsley and garlic, then cooked. After cooking they are reinserted into their shells and served.

The shells themselves are not cooked, so the baby snails that later emerged had presumably originated from eggs lodged inside the shell. These could have survived the scooping-out part of the preparation.

Gregory Sams

London, UK

The snail that is most frequently eaten throughout Europe is Helix pomatia, the species shown in the photograph. It is known in the UK as the ‘Roman snail’ because the Romans may have introduced it to these shores for food. It is native to much of Europe.

Snails that are eaten in restaurants often originate from snail farms, although they may be collected from the wild. They are hermaphrodites, but although they have both male and female reproductive organs they must mate with another snail before they lay eggs. After mating, a snail can store the sperm it received for up to a year before fertilisation, but eggs are usually laid within a few weeks of mating.

The eggs of H. pomatia are laid about 6 centimetres deep in holes dug in the soil. A snail will take up to two days to lay between 30 and 50 eggs. After about four weeks the fully formed baby snails hatch.

In this case, any eggs that were present during the cooking process would die. However, it is possible that the shell in which the snail was served was not the one belonging to its fried occupant. Snails are often supplied ready-cooked from producers with a separate supply of shells in which they can be placed for presentation, one of which, in this case, may have held eggs previously deposited in the upper whorls of the empty shell.

More information about snails and their conservation may be found on the website of the Conchological Society of Great Britain and Ireland at

Peter Topley

Bedfordshire, UK

Floundering about

How do certain animals, such as the flounder, change their colour to match their background? More specifically, if you made a tiny blindfold for the flounder, would it still be able to match its surroundings?

Nick Axworthy

By email; no postal address supplied

Many fish in the teleost group, such as the minnow, change colour in response to the overall reflectivity of their background. Light reaching their retina from above is compared in the brain with that reflected from the background below.

The interpretation is transmitted to the skin pigment cells, called chromatophores, via adrenergic nerves, which control pigment movement. Teleost skin contains pigment cells of different colours: melanophores (black), erythrophores (red), xanthophores (yellow) and iridophores (iridescent). Pigment granules disperse from the centre of the cell outwards and the area covered by the pigment at any specific time determines that cell’s contribution to the skin tone.

Many flatfish, including flounder, go further than overall reflectivity and develop skin patterns according to the light and dark divisions of their background (as in the questioner’s photograph). This seems to involve a more discriminating visual interpretation and produces distinct areas of skin with predominantly, but not exclusively, one type of pigment cell. For example, black patches contain mainly melanophores and light patches mainly iridophores, which can produce the chequerboard appearance seen in the picture.

As these responses are visual, blindfolding the fish would result in all the components of the chromatic system being stimulated equally. The fish would adopt an intermediate dark or grey skin tone similar to that on a dark night. Over time, direct light stimulation of the pineal gland through the skull will affect the amount of pigment and number of cells – a process mediated by hormones – hence the ‘black’ plaice sometimes sold in the UK, which have come from the sea around the dark volcanic seabed off Iceland.

Cliff Collis

London, UK

Many animals change the shade or even colour of their skin in response to certain stimuli. In cephalopods such as the cuttlefish, pigment-filled sacs can become extended (flattened) by the action of radially arranged muscle fibres that are controlled by the nervous system. Colour change in these animals is both rapid and spectacular.

In crustaceans and many fish, amphibians and reptiles, specialised pigment-storing cells in the skin called chromatophores redistribute the pigment inside them. The pigment in these chromatophores is either concentrated in the centre, or dispersed throughout the whole cell.

Imagine a white floor with a small pot of black paint standing in the middle. From above, the floor will look very light, despite a substantial amount of pigment seen as a small black spot in the middle. When the same paint is spread over the floor, the floor looks black. The beautiful trick of the black chromatophores (known as melanophores) is that they can reverse the process, effectively putting the paint back in the pot.

Flatfish, such as plaice, flounder and others, are expert at imitating not only the general shade of the surface on which they rest, but also patterns of dark and light material. Not surprisingly, perhaps, their eyes are used to perceive the shade and patterns. Light hitting the retina from above affects the ventral or lower area of the retina, while light reflected from the bottom strikes the dorsal or upper retinal surface.

If the light intensities from the two areas are similar, a signal causes the pigment of the melanophores to be concentrated in the centre of the cell, so the fish turns pale. On the other hand, when the bottom is dark the two areas of the retina receive very different light intensities, and the reverse of the signal causes pigment dispersion and a dark fish. The masters of disguise, the flatfish, can also discern patterns in the bottom surface and imitate them by regulating nerve activity to groups of melanophores.

Stefan Nilsson

Gothenburg University, Sweden

Don’t call me ginger

Why are orangutans orange? It doesn’t seem to be a camouflage mechanism. And why are they so hairy? They live in tropical forests after all.

Peter Webb

London, UK

Orangutans’ colouring does help them blend in. The water in peat-swamp forests, where orangutans live, tends to be a muddy orange. Sunlight reflected off this water can give the forest an orange cast, making orangutans surprisingly hard to see in dappled light. Many orangutan nests, up in the forest canopy, contain orangey-brown dead leaves, and some trees have reddish leaves, especially when young.

Ground-based predators would see orangutans in the canopy as a mere silhouette. In such circumstances orange may stand out less than black, which may be more suited to blending in with the forest floor. Dark African apes such as gorillas spend much more time on the ground than orangutans, while some other canopy-dwelling primates have a similar ruddy colour to orangutans. Among these are red langurs, which live in the same Borneo forests as orangutans.

As for orangutans being hairy, there are numerous possible reasons for this. Orangutans are exposed to direct sunlight up in the canopy, so hair could serve to protect their skin from the sun. It may also provide insulation and temperature control, trapping a layer of relatively cool air close to the skin by day and keeping the skin dry and warm at night and in cool rainy weather. Hair also protects against insect bites and helps break the outline of the animal’s silhouette in the canopy when viewed from below.

Finally, dominant ‘flanged’ male orangutans have long hair on their arms and at the base of their back. This makes them look larger, helping them dominate other males and attract females.

Mark Harrison

Orangutan Tropical Peatland Project

David Chivers

Reader in Primate Biology,

University of Cambridge, UK

Orangutans may not need camouflage, given their size and strength. And if their hair is not for camouflage, the fact that reddish hair is common in primates such as proboscis and red leaf monkeys, and in other mammals such as tree shrews, squirrels, foxes, deer and flying foxes, suggests that it isn’t an awkward thing to have.

As to whether being orange provides camouflage, that depends on the environment. Some marine species that live at depth are well camouflaged despite being bright red, because red doesn’t stand out in the low light under water. Something similar applies to orangutans.

The explanation lies in the way sunlight penetrates the forest canopy, bouncing off vegetation as it does so. Leaves absorb red, orange and violet light for photosynthesis, reflecting green. So by the time sunlight has reached the forest floor, it has been robbed of its reds and oranges. In this sort of light, orangutans look like large, dull brown lumps. They can be so well camouflaged that several times I have walked past having failed to see them sitting on the ground half a metre away. I have twice come within a hair’s breadth of tripping over them.

The explanation above is something I heard from science writer and evolutionary biologist Jared Diamond, who has studied birds of paradise in Papua New Guinea. He described males displaying in columns of dappled sunlight: as they moved in and out of the light, they changed from dull brown to spun gold, as though disco dancing under strobe lights.

Anne Russon

York University

Toronto, Canada

The writer is the editor of The Evolution of Thought: Evolution of great ape intelligence (Cambridge University Press, 2007) and the author of Orangutans: Wizards of the Rainforest (Firefly, 2004) – Ed.

Several replies noted the similarity between the words ‘orang’ and ‘orange’ – but only one explained their derivation in detail – Ed.

Orangutans may be orange, but the name has nothing to do with their colour. It comes from Malay and means ‘person of the forest’, orang being Malay for ‘person’. The word ‘orange’ – originally meaning the fruit, but later used for the colour too – came into English via a trail of other languages.

It ultimately comes from India (Tamil via Sanskrit), from where it made its way into English in the 15th century via Persian, Arabic, Italian and French. So ‘orangutan’ and ‘orange’ are unconnected etymologically.

Linguistic coincidences are quite common, as it happens. The word for ‘dog’ in Mbabaram, an indigenous Australian language, is dog, although the word is not borrowed from English. Similarly, the word for ‘honey’ in both Hawaiian and ancient Greek is meli. Chance similarities are a major problem in establishing whether two languages derive from a common ancestor.

Sometimes these coincidences take on a life of their own. At one time a berfry was a tower, but when people started to associate the first part of it with the word ‘bell’, it changed to ‘belfry’ and started to mean a tower with a bell in it. So far, no one is claiming that orangutans are so named because they are orange, but it’s not inconceivable that one day someone might.

David Willis

Department of Linguistics

University of Cambridge, UK

Catching the red eye

Some bird species, such as the great-crested grebe, hunt underwater for fish and have red eyes. The red colouring is presumably beneficial to these diving birds, but in what way? If it does provide an advantage, why have other birds with similar habits not evolved red eyes?

Ian McKechnie

Weybridge, Surrey, UK

The red eye is caused by the colour of the iris, which controls the diameter of the pupil. Only some diving birds have red irises. The eye of a king penguin, for example, is black, and set against black plumage.

The pigmentation of the iris is just a device for making the iris as opaque as possible to accurately control the amount of light reaching the retina, by ensuring no light leaks through the iris muscles. Its colour is in this sense irrelevant.

But eye colour was extensively reviewed by ophthalmologist Ida Mann in 1931, who concluded that iris colour is used as a signal between animals. There has been no evidence to counter this conclusion since.

So that a structure used for signalling is conspicuous, it needs to contrast with its surroundings, and so in some birds – such as the great-crested grebe mentioned in the question – bright iris colours often contrast strongly with the feather or skin of the head in birds (or, in certain cases, they match it).

Eye colour varies widely. There are grebes with yellow eyes, penguins with black, red or yellow eyes, and cormorants with blue, green, red or black eyes.

Iris colour changes with age in many birds, with the young showing only browns and blacks and not attaining the full bright colour until adulthood. Presumably this is when they need to use iris colour as a signal of fitness or when emotional state becomes important.

Graham Martin

Emeritus Professor

Centre for Ornithology

School of Biosciences

University of Birmingham, UK

Tiger, tiger

Why do tigers have stripes? The other big cats tend to have spots.

Linda Veron

Tarragona, Spain

The beautiful striped markings on tigers’ coats are unique in the cat family. Other closely related big cats have spotty rosette or cloud-shaped body markings (leopards, jaguars and clouded leopards), or plain coats (lions).

Work by our team at the University of Bristol has shown that cat patterning evolved to provide camouflage suited to the cats’ particular habitats and behaviours, enabling them to capture prey more effectively (and escape predation in the case of the smaller cats). In general, plainer species such as lions live in open environments and hunt by day, whereas cats with complex patterning like leopards and tigers have more nocturnal habits and live in environments with more trees. Our statistical analysis nicely supports commonsense natural history.

Unfortunately, with no other striped cats around besides tigers, we cannot use the same methods to identify the evolutionary factor which drove tigers to depart from the ancestral big-cat pattern. Tigers are much bigger than jaguars and leopards, but in general they have a similar ecology, and tigers’ historical range and habitat overlap considerably with those of leopards. So why don’t they look similar?

One idea put forward decades ago, but for which evidence is still lacking, is that compared with the typical leopard habitat, the average tiger habitat contains a lot of vertical features such as bamboo. Quantifying this would be straightforward except that with the tigers’ range now so shrunken, it is hard to know exactly what sort of forest their coat evolved in. Tigers are obviously well camouflaged, yet the factors behind their appearance remain an enigma.

Intriguingly, our team has also shown that big-cat patterning changes relatively rapidly over evolutionary timescales. One day our descendants might wonder at the beauty of striped leopards and spotty tigers.

Will Allen

University of Bristol, UK

Your earlier correspondent’s discussion of the tiger’s stripes was excellent. However I must take issue with the comment ‘… with no other striped cats around besides tigers …’. The tiger may be the only large striped cat, but there are a number of smaller striped wild cats. The European wild cat (Felix Silvestris) and its subspecies are striped. The sand cat (Felis Margarita) is striped, and the fishing cat (Prionailurus viverrinus) has both stripes and spots that vary in dominance depending on the individual cat.

David S. Rubin

By email; no postal address supplied

2 Ice, bubbles and liquid

Air spray

I wanted to chill a mug of water so placed it in the freezer, but then forgot about it and it froze solid. When I removed the block of ice from the mug it contained the most amazing thistle-like pattern of what seemed like canals of air. None of these canals extended to any outside surface. What happened?

Brian Barnes

Somerset West, South Africa

In the freezer the water cools from the outside in. So the first crystals of pure ice form around the outside of the mass of water. As these ice crystals grow inwards, the air dissolved in the water becomes trapped and its concentration increases. Initially it remains dissolved, because cold liquids can hold more dissolved gas than hot ones. This is why the outer layer is almost free of air bubbles.

However, the concentration of dissolved air eventually exceeds the ability of the water to retain it in solution, so air bubbles begin to form and are trapped in the ice as the crystals grow inwards, forming the patterns observed. The lines all curve downwards because water on the verge of freezing is less dense than slightly warmer water, and so rises. Thus the water freezes from the top down, meaning that the water at the bottom (which is also somewhat insulated from the cold air in the freezer) is the last to freeze.

Simon Iveson

Department of Chemical Engineering

University of Newcastle

Callaghan, New South Wales, Australia

If the container is smooth, the outer layer is bubble-free because solutes in the water, gases in particular, are unsaturated when freezing starts. Bubbles form only after some ice has formed, forcing enough gas into the surrounding water to supersaturate it. Initially, tiny spherical bubbles appear on the advancing surface. What happens next depends on the path of the ice front and the form of the growing crystals.

In this case, the front advanced smoothly along the curves described by the long bubbles. The original spherical bubbles caught in place by the growing crystals acted as nuclei into which the rest of the gas collected as it escaped from solution. Being surrounded by ice in every direction except perpendicular to the ice surface, the bubbles formed tubes whose shapes traced the advance of the crystal-water interface.

It’s common to see such bubble growth, but the lovely symmetry and consistent bubble structure shown in the photograph require still water and suitable solutes, temperature gradients and crystal forms. To sculpt such bouquets in various forms, and possibly in other media, should make for satisfying science experiments and art projects.

Jon Richfield

Somerset West, South Africa

N factor

Leaving the house early one frosty morning, I noticed the emblem on my car was frosted over on all the letters but one. The N was completely frost-free as you can see in the photo. The outside temperature was hovering around 0 °C. If all the letters were constructed of the same material, surely they would all appear the same, so why is the N special?

Nikki Cherry

Peterborough, Cambridgeshire, UK

The N is frost-free because it is in poor thermal contact with the car body, perhaps because it is slightly loose or there is a small gap behind it.

All objects cool by emitting infrared radiation. Metals cool more effectively in this way than plastics because metals have a high thermal conductivity, which means it is easier for heat inside a metal object to move to the surface and escape.

As this cooling occurs, the surface of an object initially becomes cold enough for water to condense on it (take a look at a cold beer bottle on a humid summer day). When the ambient temperature is close to freezing, the surface may eventually get cold enough for that condensed water to freeze, or for water vapour to condense directly onto it as small ice crystals. Both of these processes give rise to frost.

The letters in the manufacturer’s logo are made of plastic. If they were somehow floating just above the car body, they would all stay frost-free because plastic cannot radiate heat fast enough for them to frost over. It is because they are in contact with the car body that they can lose additional heat to the metal via conduction, allowing them to get cold enough for frost to form.

The N must be in poor contact with the metal for some reason, hence the absence of frost, though you can just make out it has become cold enough for water to condense onto it.

Adam Micolich

School of Physics

University of New South Wales

Sydney, Australia

Cranberry ice

One of my faculty colleagues, Michael Runtz, took this photo of ice bubbles in Cranberry Lake in Ontario. How did the bubbles form in this amazing fashion?

James Cheetham

Department of Biology

Carleton University

Ottawa, Ontario, Canada

Without any scale reference or indication of the depth of the lake I cannot tell for certain, but I have seen similar bubbles frozen in ponds where I grew up in upstate New York.

In freshwater ponds and lakes, the biological activity of microbes in the sediments on the lake floor produces bubbles of gas, usually methane or carbon dioxide. In winter this activity is slow, but it is still present.

The gas bubbles rise to the frozen surface of the lake, becoming trapped there. The following night, another layer of ice forms beneath the bubble, so it is encased in ice. This leads to the flattened shape you see. The picture is a frozen daily record of the gas emissions.

Obviously you need calm, shallow-water ponds or sheltered water at the edges of small lakes in order for this phenomenon to occur.

James Field

Aberystwyth, Ceredigion, UK

Crossed channels

While cycling in Ireland I had ample opportunity to observe rain and puddles, and I took this photo of muddy water running across a road. Why has it separated into bands, and what determines their spacing?

Robert Johnstone

Leeds, UK

What you see here is the effect that changing the velocity of a fluid has on any solids being transported by it. If the fluid is moving with a velocity above that at which the solid would normally settle out, localised currents in the fluid will stop the solid settling normally. I suspect the road has some surface imperfection which caused an initial drop in velocity, allowing some of the sediment load to settle onto the road surface as a band. The second band is then caused by the drop in velocity generated by the first band and so on, with the spacing determined by the gradient and roughness of the surface. The bands are curved because the water eventually builds enough pressure to force its way around the sides of each band of sediment. With this pressure comes velocity and so, once the velocity is high enough, no more sediment settles out in that band.

The same principle of solids settling because of a drop in velocity is used to separate minerals such as gold from river sediments using a pan.

Neil Ayre

Kalgoorlie, Western Australia

This is a result of a complex series of interactions involving, among other factors, the speed of the fluid flow, the viscosity of the fluid, and the ratio of the thickness of the top layer to the overall thickness of the fluid.

This and related problems were first studied by John Scott Russell in the early 19th century. In the later part of that century Diederik Korteweg and Gustav de Vries modelled the phenomenon with an equation that bears their names. The types of self-reinforcing waves formed in this situation are known as solitons and they are fundamentally different from normal waves occurring in bodies of fluid such as the sea, where the depth of the fluid is very large compared with the height of the wave crest.

Doug Dean

Pfeffingen, Switzerland

This occurs when water or other fluids run in a thin film over a surface. Male readers may sometimes see the effect when using urinals – Ed.

The waves shown in the photograph are a classic example of what are known as roll waves. Other examples include the ‘urinal effect’ noted by your (male) editor and also rainwater flowing down a window (which is effectively the same thing). On a larger scale, roll waves sometimes appear on spillways discharging the overflow from reservoirs, where they can become large enough to overtop the sides of the channel that would comfortably contain a uniform, steady flow. In extreme cases the flow effectively moves in surges with very little water in between, which gives the phenomenon another name: slug flow. Roll waves occur on the free surface of liquids, liquids carrying solid particles in suspension, slurries, and also at the interface of immiscible liquids such as oil on water.

Contrary to what your earlier correspondent suggests, they are quite different from solitons, which are essentially discrete pulses of liquid moving on top of the otherwise undisturbed flow beneath them, and nor are they miniature waterfalls over static bands of silt. If these existed, they would obscure the line in the road running from top right to bottom left of the photograph.

Roll waves have been studied for more than 80 years. For a gentle slope the flow will be slow and deep – what is known as subcritical flow – while for a steep slope the flow will be shallow and fast-moving – or supercritical. The difference between these two states is given by the Froude number, named after the 19th-century fluid dynamicist William Froude. This number is the ratio of the velocity of the flow to the speed of very small waves that invariably appear on the liquid’s surface, and can be greater than or less than 1. For supercritical flow it is greater than 1, which means that the waves move slower than the flow and therefore can only travel downstream.

The speed of waves increases with their height, so larger waves will overtake and absorb the smaller ones (which also increases the speed of the large ones). Gradually, the many tiny waves become fewer, larger ones. Eventually the flow in their vicinity becomes subcritical, the wave fronts steepen and they break in much the same way as waves break on a beach.

Roll waves do not appear at regular intervals and there is no way of calculating the average distance between them. They appear spontaneously even when the flow is over a smooth surface, provided the Froude number is greater than 2. A slightly rough surface appears to promote their appearance, but further increasing the roughness has the opposite effect and ultimately will prevent them occurring altogether.

Richard Holroyd

Cambridge, UK

Frozen images

In January I dropped some bricks into my pond, which is a metre deep. In March the pond froze over and an image of the bricks appeared like a hologram in the ice – as shown in the photo. What caused this?

Clive Gardner

Glenfield, Leicestershire, UK

Pond water contains a certain amount of dissolved gas, including oxygen. Because of the physical properties of water, the colder it is the less gas per unit volume it can hold. Water is at its densest at a temperature of about 4 °C.

As the water temperature in the pond drops, cooled by the colder air above, the surface water sinks to the bottom by convection. Once the whole body is at 4 °C this convection stops because further cooling makes the surface water less dense. The surface starts to freeze and the coldest water begins to release its dissolved gas. Some of this would bubble upwards, but much more would diffuse down, still remaining in solution, until eventually the water surrounding the bricks becomes supersaturated.

The rough surface of the bricks, particularly around the edges and corners, provides nucleation sites for dissolved gases. Gas molecules collect preferentially around the edges of the bricks, eventually producing bubbles. As these reach a critical size they break away and float straight upwards in the still water. Because there is a layer of ice on the surface, the bubbles become trapped and frozen into it. As the ice layer thickens and bubbles continue to rise from the brick, the 3D shape develops. The rate of bubbling was probably very slow, as was the rate of freezing, which allowed the very detailed effect to form.

David Jackson

Liverpool, UK

A previous correspondent states: ‘Because of the physical properties of water, the colder it is the less gas per unit volume it can hold.’ Actually, the opposite is true: water holds more gas as it gets colder. This is why opening a bottle of fizzy drink on a hot summer day releases more fizz than opening one straight from the fridge.

What does happen is that water releases most of its dissolved gas when it freezes. In the case of this question, the layer of water on top of the pond releases bubbles of gas when it freezes, which is why the water below becomes supersaturated with gas.

Tim Patru

By email; no postal address supplied

Ice vines

Living in an older home with single-glazed windows, I have grown used to seeing intricate patterns of frost on the panes each winter. However, I was truly impressed by this twining, vine-like pattern that appeared one January, and I would love to know how it came about. The vine-like shapes formed in a 20-by-30-centimetre section of the window and were surrounded by standard snowflake-shaped frost. The ‘vines’ were 1 centimetre wide with small dots running up the centre, and they twisted about each other with leaf-like shapes sprouting from the sides. The photo below shows a section measuring about 6 by 10 centimetres. It had been a particularly cold day (–20 °C) and the sun was shining on the window. My wife suggested that the sunlight shining through the branches of a tree 2 metres away had caused this, but there were no distinct shadows visible on the window at the time.

Ken Zwick

Neenah, Wisconsin, US

No clear answer to this one, but some hypotheses. This phenomenon has been seen by many people – Ed.

During the winter my conservatory roof is regularly covered with an even layer of frost crystals in no particular pattern, but on three occasions the entire 20 square metres of double-glazed glass has been covered with ‘ice vines’ (see photo below). Because there are no trees anywhere near my conservatory the patterns could not have been formed by sunlight shining through branches.

The ice vines seem most likely to form on frosty evenings after mild, sunny winter days when air pressure is high, the sky is clear and there is a gentle breeze. At these times, bands of condensation form on the glass with dry stretches between them.

I assume that the breeze flowing over the conservatory frame, which stands about 4 centimetres above the glass, sets up an oscillation in the air, rather like the waves produced in a wind instrument such as a recorder or a whistle. As the air touches the glass, chills it and bounces off again, this may form the bands, which always run parallel to the frame, although the width of the bands varies with wind speed. Generally, the ‘stems’ of the ice vines follow the direction of the frame, so it appears that the bands of condensation and the ice vines are somehow linked.

When growing crystals, the finest and largest specimens form when the crystals grow very slowly. It is possible that the gently oscillating air over the conservatory roof reduces the rate at which water molecules crystallise, adding to the complexity of the ice patterns. Also, as the breeze dies away during the night, cold air sliding down the slope of the roof may change the direction of the oscillations, causing even more intricate patterns.

Steve Antczak

Lymington, Hampshire, UK

I have a picture of an almost identical pattern on my car’s rear windscreen (see below) after it was parked overnight away from any trees during frosty weather.

It is a very striking pattern. It looks as though something has progressed in a stuttering way along the vines. I have had the same car for several winters but have spotted this effect only once.

Alan Singlehurst

Shildon, Durham, UK

We received further photographs (one is shown below) of this astonishing phenomenon from reader Steve Redpath of Aberdeenshire, UK, who spotted ice vines on his roof window last winter – Ed.

Snow squares

This photograph was taken mid-morning after a light fall of crisp snow in the night. The temperature had not risen much above 0 °C. I have always assumed that the circles form because the concrete slabs conduct heat differentially from the open gaps between them. But what causes the centres to melt?

Neil Howlett

Frome, Somerset, UK

Working on the assumption that 150 correspondents can’t be wrong, we selected the three following answers – Ed.

The pattern is caused by the five-blobs method of slab laying, in which a blob of mortar is placed under each corner of the slab, and one at the centre. This makes it easier to level the slab. These blobs are conducting heat from the ground underneath, melting the snow in this pattern.

Steve Law

Kingston, West Sussex, UK

Regular viewers of TV gardening programmes will know that the preferred method for laying a patio is to lay the slabs on a full bed of mortar, so that the slab is supported across its entire undersurface. Laying on a full bed will not only stop this pattern showing up, but will also ensure that ants don’t make a home in the gaps under your patio slabs.

Gary Stanley

Castleford, West Yorkshire, UK

With a bit of foresight, a creative builder could produce an effect whereby interesting patterns, pictures or words could appear as a light snowfall melts.

David Turner

Sevenoaks, Kent, UK

Ring of bright air

While watching beluga whales at Vancouver aquarium I noticed that one of them was blowing air rings. These appeared as annular bubbles that exhibited no obvious buoyancy and could be propelled to the bottom of the pool. Some of the whales would blow one horizontally then swim over to it and suck it back in. Have there been any studies of these intriguing air rings? How fast can these bubbles be projected and to what depth and does it vary between animals? And why do they produce this shape – does it have any beneficial purpose?

John Chapman

North Perth, Western Australia

Unlike soap bubbles in air, air bubbles under water are maintained by the pressure of the surrounding water rather than tension. The beautiful bubble rings blown by dolphins are actually toroidal vortices that they create by sending a localised gush of water into an otherwise still region, which they then inject with air. This air moves to the centre of the vortex because that is where the pressure is lowest.

Stir water rapidly in a cylindrical glass with a smooth rod (to avoid too much turbulence) and you may see the centre of the vortex sharply depressed where the pressure is lowest. If you stir faster, bubbles may detach and move down the core of the vortex, illustrating the behaviour mentioned above.

Steve Gisselbrecht

Boston, Massachusetts, US

A ring-shaped bubble is an example of a vortex ring, which is similar to a columnar or tornado-like vortex, but bent round into a circle. Other examples include smoke rings. They are formed when a flow through a circular opening is forced back on itself.

There is also a second class of bubble ring. These consist of water vapour in water, and form when a vortex ring generates low enough pressure to cause vapour to form around the ring of the vortex, giving it a similar appearance to an air ring under water, though filled with vapour. This type of ‘cavitating’ vortex is used by some underwater weapons systems.

David Hambling

London, UK

Cavitation vortices can be seen spinning off the tip of ships’ propellers, and they form when the liquid flows so rapidly that its pressure drops enough for it to turn into vapour – in essence the water boils underwater.

There is another even more exotic way to create such bubbles. Reader David Williamson of London points out it is possible to create ‘antibubbles’. An antibubble is a bubble in reverse. Just as the soap bubbles that children blow are a thin skin of liquid in air, an antibubble consists of a thin skin of air with water inside and out.

Their properties have been explored by S. Dorbolo, H. Caps and N. Vandewalle of the Institute of Physics at the University of Liège in Belgium, and described in a paper published in 2003 in New Journal of Physics. You can find out how to make antibubbles at

Alex Vallat of Cambridge, UK, tells us that blowing air rings underwater is not difficult to do. Puff your cheeks out with your lips pursed. Then, with your throat closed, make a P sound with the lips and use the stored air to blow out quickly (see photo on previous page). The tube that forms the bubble rotates around its core, like the smoke rolling around a smoke ring, but the ring itself does not spin around its centre like a steering wheel.

Steve Backshall of Wooburn Green, Buckinghamshire, UK, has a different approach. Try sitting on the bottom in relatively still water, he says. Then rock backwards so your mouth is facing upwards, place your tongue firmly on your upper lip, then forcefully expel air before briefly sucking back in and closing your lips. Accomplished ring blowers can create mesmerising, expanding doughnuts of shimmering air that gyrate towards the surface (see below).

Dolphins blow toroidal rings for fun and then play with them underwater. Reader Alistair Eberst of the University of Abertay Dundee in Scotland drew our attention to, where you can see pictures similar to the one below, and much more – Ed.

In 2011 The first ‘King of the Bubble’ contest was held in a specially built 33-metre deep pool in Brussels, Belgium. Contestants had to produce the largest, clearest, longest-lasting, perfectly rounded ring bubbles in order to take victory.

Peter Mann

Ruislip, Middlesex, UK

One answer sometimes leads to another question. We have heard numerous anecdotes of dolphins, whales and even human swimmers who have been able to create stable toroidal bubbles capable of existing underwater. And the questioner below wants to know if they can be created in any environment. Do any of our readers know what the secret to these bubbles is? – Ed.

Soap on a hope

Is it possible to blow a toroidal soap bubble (one shaped like a ring doughnut)? And if it is, would it collapse immediately to a sphere? Could its life be prolonged by spinning its surface, as with smoke rings?

Peter Gardner,

Blawith, Cumbria, UK

A soap bubble is the minimum surface which encloses a given volume. If a toroidal bubble were created, it would not provide such a minimum surface and would therefore tend to contract to reduce its surface area until it collapsed into a bubble which would then burst because of the forces created at the disappearing hole in the torus. This situation differs from that in a solid torus such as a bicycle inner tube, because soap bubbles can transfer part of their surface from the inner to the outer part of the torus as they shrink.

A temporary toroidal bubble could perhaps be created by sticking spherical bubbles in a ring and collapsing their shared walls, but the inner ring would undoubtedly degenerate as the number of bubbles decreased.

Soap bubbles are different from smoke rings, which have no surface but are composed of solid particles suspended in air. These are stable because different parts of the body can rotate at different speeds without causing degeneration.

Jerry Humphreys

Bristol, UK

As a mathematician who studies soap bubbles, I knew that a toroidal soap bubble was, under normal circumstances, impossible. The only stable equilibrium shape for a soap bubble is the sphere that most people easily recognise – a torus bubble should not even exist in unstable equilibrium.

So when the famous performer Tom Noddy (known as the Bubble Guy from the US TV show Tonight) told me that he once blew a toroidal bubble, I didn’t actually believe him until he showed me the photographic proof (below). The bubble didn’t last long, but it did exist briefly. Visit to see some further interesting examples.

Torus bubbles do occur in unstable equilibrium in double soap bubbles: an outer bubble wrapped around another at the centre, as in the diagram below – a copy of a computer simulation created by John M. Sullivan, Professor of Mathematics at the University of Illinois. More of his images are online at

Frank Morgan

Williams College

Massachusetts, US

Woodland wonder

The ice crystals in the photo were found on small branches lying on the ground in mixed woodland. The crystals had formed only where the bark was missing and there was very little frost elsewhere on the ground. Can anybody explain how they formed?

David Meadows,

Yeovil, Somerset, UK

The explanation behind this photograph of fine, frilly ice filaments growing from small branches is that the hairs of ice are generated from water held in the pores of the decaying wood, which is sucked out by the freezing action to form filaments. For this to happen there would have to be no free water on the outside of the wood. Indeed, the questioner notes that there was little frost elsewhere on the ground.

I have seen the same ice crystals on rotting elder wood alongside the river Barle, downstream of Wimbleball dam, also in Somerset.

In the example in your questioner’s photograph the moisture held in the pores of the wood was still liquid and was being extruded and frozen at the surface. The fibres reflect the size and spacing of the pores in the substrate. Their curvature was caused by the weight and drag at the point of freezing (like meat coming out of a mincer). When the temperature differential is low, ice crystals are not able to penetrate fine pores and are extruded instead. Water will even be extruded as ice crystals from between the minute particles in glutinous sludges, leaving a more crumbly and concentrated residue. This was the basis of a process once used for treating waterworks sludges.

A more common phenomenon is columnar ice, which can be seen on chalk downs, for example, where a layer of ice grows out of damp porous lumps of chalk in frosty conditions.

David Stevenson

Newbury, Berkshire, UK

Miracles have no explanation, but magic does and, in this case, bottles of milk left on an icy doorstep provide the model. On two mornings this winter I found some 10 centimetres of frozen milk extruded from my pint, curved to some extent and still wearing the aluminium cap. The remaining liquid was in the form of a slush. I should add that this effect was very common 50 years ago before global warming set in.

For this phenomenon to occur on soil and rock requires a source of water held in pores to cool gradually from 4 °C to 0 °C, or perhaps lower under pressure. As some of the water molecules begin to form short-range crystalline structures, the water expands. Under these conditions, ice fibres are extruded through the pores. Gravity and the phenomenon of regelation – when partially thawed ice re-freezes – may cause the curvature. The ambient conditions described by your correspondent were perfect.

Philip Sutton

Gateshead, Tyne and Wear, UK

Wrapped bubble

I found this strange narrow spiral bubble trapped in a frozen puddle in Weardale, County Durham, UK. Nearby puddles of about the same size had typically a dozen or so irregular sausage-shaped bubbles, usually wider than this spiral but displaying a similar concentric nature to varying degrees. The one photographed was the most striking, however. How and why did it form?

Bob Johnson

Durham, UK

In this case, dissolved gas is not the issue. The ‘bubbles’ are the result of the puddles sitting on porous ground.

As the puddle begins to freeze, the water is still draining away under the ice. If the ice cover is thin enough the unsupported surface sags. A meniscus can form between the ice cover and the water below, and this then freezes to form a bridge to the ground.

The freezing tends to draw water up from the ground through these menisci because they conduct heat better than the air of the bubbles in between. The result of this process is usually a series of large, flat, frozen air spaces, but in this case there has been an unusually long progression of a freezing meniscus that has followed the line of contact with the receding water under the concave ice cover.

The rate of freezing and the rate of drainage of the water must have been just right for this spectacular spiral figure to form.

David Stevenson

Newbury, Berkshire, UK

Tank trap

The Plum Temple in Zhaoqing, near Canton, in China, features an optical illusion tank (below) about 4 metres long and a little under a metre across. The puzzle is in the perceived depth. If you stand at the right-hand end, the water appears to be about 1 metre deep, sloping away to the left end where it appears to be about 10 centimetres deep. If you move to the left end and look to the right, then the appearance is reversed and you are now at the deep end. If you stand in the middle, the bottom appears to be U-shaped, with the deepest point exactly in front of you, sloping away to become shallow at both the left and right ends. When I visited, not even the technical types in our party could explain it. The temple was built in the 10th century, so this tank has been baffling visitors for more than 1000 years. Can anyone offer any explanation?

Phillip Bruce

Hong Kong

The optical illusion shown in the reader’s photograph is simply a consequence of the bending, or refraction, of light as it leaves the water in the tank.

Light travels at different speeds depending on the medium it is in, and it travels faster in air than in water. The change in speed causes a ray of light to be bent where the water meets the air. Assuming a flat water surface, a ray of light coming from beneath the water at the far end of the tank will be closer to being parallel to the water surface than a ray coming from directly underneath the observer. The nearer the ray is to being parallel to the water surface, the more it bends towards the observer. But the observer’s eyes do not account for the bending and assume that the light is travelling in a straight line. This makes it seem as if the ray is coming from a point that is shallower than the true point of origin.

Therefore, the closer part of the tank’s floor will appear to be roughly at its ‘true’ depth, whereas the far end of the tank will be seen as shallower.

Using computer ray-tracing software, I have recreated the scene as best I can (see photographs opposite), using the correct optical properties of water. You can see the view from the right-hand end of the tank and the view from the middle of the tank. These images were rendered using a free ray-tracing program called POV-Ray ( This program takes a description of a scene, written in its own peculiar language, and draws it by tracing the path of light from its final position in the image back onto the objects in the scene, while taking into account effects such as reflection, refraction, fogging, filtering and emission. This is computationally intensive, and despite the simplicity of this scene, both images took over two minutes to render on my ageing 233-megahertz Pentium-II PC.

Beginning with the estimated size of the tank given by Phillip Bruce, I constructed a hollowed-out box for the tank, and placed another smaller box inside it. The smaller box was given the optical properties of water (95 per cent transmission, index of refraction of 1.33), except that I reduced the amount of reflection in order to show the tank floor more clearly. An infinite plane creates the floor, and another box creates the back wall. A soft light was added. POV-Ray is unable to produce brick-like textures, so I gave it a chequered pattern instead. Many minor adjustments were needed to get the cameras into suitable position.

Andrew McRae

Queensland, Australia

Refraction is part of the explanation. The bending of light at the water surface makes the tank’s flat bottom look curved. However, this does not explain the fact that the cement lines in the tank wall’s tiling are straight.

In fact, these tiles are not under the water but are the reflection of the wall above the tank off the water’s surface. The tank walls are dark, probably the same colour as the wall above, but they are obscured by the reflection of the upper wall. Hence you see the reflected upper wall with straight cement lines, as these lines are not influenced by refraction.

The base of the tank is light coloured so that it reflects more light than the upper wall surface reflection, and hence we see the brighter image – the principle of the one-way mirror. If your enquirer were to return to China and paint the bottom of the tank dark grey, the illusion would be lost and only the reflection of the back wall and the signs would be visible.

Doug White

Dogmersfield, Hampshire, UK

The illusion is a more extreme form of the optical distortion present in any swimming pool, and is strengthened by the observer (like your photographer) having his eye closer to the water surface. It can also be strongly reinforced if the refractive index of the liquid in the tank is increased, as will happen if its density is increased by adding solutes such as salt.

The dense solution could be overlaid with fresh water to yield a stepped density gradient which would increase the illusion. Such gradients resist thermal convection, and are stable enough to be used in Israel for solar collection ponds near the Dead Sea.

I wonder if the mottling of stone and brick is because of the presence of salts or other solutes in the tank.

J. O. N. Hinckley

London, UK

Frozen veg

I took this picture (below) of ice in a field earlier this year. Individual grass stalks are encased in the ice. Nearby was an irrigation system that emitted a fine spray at certain times of day. Are the two connected?

Liz O’Neill

By email; no postal address supplied

Your correspondent is correct in supposing that the fine spray of the irrigation system produced the near-vertical icicles on the grass blades. The photo on the next page shows a similar phenomenon. This one was taken at an industrial plant in south Wales in 1953. The leaking fire hydrant on the left of the picture produced a fine spray which froze into icicles on the nearby brick wall. Odd that I should wait 60 years before finding a use for the photo…

David Gregorie

Waikanae, New Zealand

The two are certainly connected as I saw a similar phenomenon in Iceland earlier this year. The Gulfoss waterfall was throwing up a fine, cold mist of spray which condensed on blades of grass and then froze in the cold air. This created ice-enclosed grass similar to that described.

Charles Harrison

Portsmouth, Hampshire, UK

I took this photo (opposite, at top) in a field near my house a couple of years ago. The pipe running into the water trough was emitting a continuous fine spray. I presume the reason such fine blades of grass remain standing with such a weight of ice on them is due to the fact that the ice was deposited in many thin layers.

Sophie Yauner

Albury, Surrey, UK

Undoubtedly the irrigation system was responsible. A few years ago I took this picture (opposite, at bottom) of a water plant in my outdoor pond. It was close to a fountain and repeatedly splashed with water during a period of sharp frost. As you can see, the water froze onto the plant stems and formed these amazing, golf ball-sized spheres.

Barry Soden

Bexhill-on-Sea, East Sussex, UK

Because a limited area is affected, the sprayer is almost certainly responsible. The ice is so clear because fine droplets formed delicate films, first on the individual blades, then on wet ice barely below 0 °C. The freezing water film is so thin that gases coming out of solution escape instead of forming misty bubbles in the ice. Water supply rate, temperature and humidity are too high for droplets to freeze while still in the air, or for hoarfrost crystals to form, so we get those clear individual ice lollies.

A similar effect is important in protecting frost-sensitive fruit trees in places like the South African Great Karoo. When night temperature and humidity in still winter air drop below critical levels, then farmers use sprayers. This prevents the dreaded dry ‘black frost’ that forms at temperatures below 0 °C destroying the crop for the coming season. Instead, the spray freezes gently into a protective layer of relatively ‘warm’ ice on the buds and twigs. Latent heat released when water freezes keeps the temperature close to 0 °C.

Jon Richfield

Somerset West, South Africa

3 Clouds and stuff in the sky

Clouding the issue

On holiday in Taormina, Sicily, about 30 kilometres north-east of Mount Etna, which we could see from our window, we awoke at 6.45 am to see an odd cloud drifting towards us. What caused it? Was Etna responsible? My grandson suggests it’s a flying saucer, but I’m sure that’s not it.

Joyce Lowe

Newtown Linford, Leicestershire, UK

The clouds shown are lenticulars, which are caused by waves in the air downwind of mountain ranges. Lenticulars do not drift, but form continuously as moist rising air from the upwind side of the mountain condenses. As the air descends on the downwind side it warms and the cloud evaporates. Any observed movement of the cloud is in fact caused by a change in wavelength.

Solitary mountains rarely produce waves strong enough to form clouds, but in this case airstreams deflected around each side of Mount Etna may meet on the downwind side and contribute to the updraft. Lenticulars are often best seen in the early morning before thermals that form during the day disrupt the wave system.

Andrew Brown

Glider pilot

London, UK

The formation is a lenticular cloud, or to give it its technical name, an altocumulus standing lenticularis, and is almost certainly connected with nearby Mount Etna. This is not because it is a volcano, but simply because it is a high mountain close to the sea. When stable moist air, such as wind blowing off the warm Mediterranean sea, flows over mountains, a series of large-scale standing waves can form on the leeward side, and lenticular clouds can form at their crests.

Not only are lenticular clouds striking to look at, they also provide useful signposts for aviators, albeit for quite contradictory reasons. Pilots of large aircraft try to avoid lenticular clouds because of the threat posed by the extremely powerful rotor forces that fashion their distinctive shape. Glider pilots, on the other hand, will actively seek out ‘lennies’ to use those same vertical air movements to obtain lift. Indeed the current altitude and distance records for gliders were set employing this so-called ‘wave lift’.

It has been claimed that the phenomenon played a decisive role in the 1942 battle of the Coral Sea, which lies between Australia, Papua New Guinea and the Solomon Islands. A force of elderly US Navy Devastator torpedo bombers, hunting the Japanese fleet and low on fuel, found their path blocked by Papua New Guinea’s Owen Stanley mountains. At almost the point of no return Commander W. B. Ault, the formation’s leader and an experienced glider pilot, identified the distinctive cloud patterns associated with wave lift and used them to enable his squadron to soar over the range, where they found and sank the carrier Shoho, the first major Japanese warship sunk during the Second World War, in what proved a turning point in the Pacific war.

Your reader’s grandson is not the first to propose ‘flying saucers’. Veteran UFO debunker Donald Menzel, and the US government’s Condon report – which ruled out the existence of UFOs back in 1968 – have pointed out how frequently this mistake is made.

Hadrian Jeffs

Norwich, Norfolk, UK

On the tube

I photographed what looked like a huge tube of cloud floating just below a uniform blanket above rural Oxfordshire, UK, at 7.30 am on 11 December 2007. Anyone know why it formed?

Shuvra Mahmud


Despite resembling a cigar rather than being saucer-shaped, such clouds are formed by a variant of the same process as that described in the previous question.

Like the slightly more familiar circular lenticular clouds, cylindrical clouds are fashioned by rotor currents, when air rising through a region of high humidity abruptly descends once more and the water condenses out to form droplets. Usually this takes place along the lee side of mountain ranges or high ridges, such as those found in the Owen Stanley range in Papua New Guinea and along Germany’s Rhine valley. Circular clouds form when the currents mould the water droplets like clay thrown onto a spinning potter’s wheel, while a cloud cigar such as this is shaped like a length of dough being rolled on a pastry board.

Although cloud cigars are most common over mountainous regions, they also appear in areas where there is no obvious topographical cause for rotor currents. They have been frequently seen over the Netherlands which, like rural Oxfordshire, is not an area noted for its mountainous terrain. So it seems possible that cloud cigars may also be created by microclimatic effects such as convection currents generated by heat from large urban or industrial areas.

The best places to see cloud cigars are the Massif Central in France and the Alps, the former being a hotspot for sightings during the great French UFO flap of the mid-1950s, and the latter the location for numerous reports of huge cylindrical ‘foo fighters’ by British fighter pilots during the Second World War. It is easy to imagine how the seemingly smooth surface might lead a witness to believe they are observing an artificial object. The giveaway is that almost all UFO reports of cloud cigars mention a vaporous exhaust, which is simply caused by wisps of cloud breaking from the main body.

Hadrian Jeffs

Norwich, UK

Sky sports

How do clouds like this form?

B. J. Baxter

Ilford, Essex, UK

The photograph appears to have been digitally manipulated to enhance the effect. If it has not, the phenomenon depicted is exceptional in its intensity – Ed

This cloud form is known as mamma (the Latin word for breast or udder) and is technically described as a ‘supplementary cloud form’. It is created when downdraughts bring cold air from higher levels, causing the air to reach its dew point and condense into cloud droplets. Compensating warm air rises between the individual pouches of falling air.

Mamma can form beneath various cloud types, including cirrus, cirrocumulus, altocumulus, altostratus and stratocumulus, where they often appear irregular in shape. However, beneath the overhanging ‘anvils’ of cumulonimbus, where heat has been lost to the atmosphere from the top of the anvil, they are often sharply defined pouches, as shown here. Mamma sometimes take the form of long contorted tubes that resemble the intertwined trunks of elephants.

Storm Dunlop

Chichester, West Sussex, UK

The pendulous features at the base of the cloud appear to be mamma (also known as mammatus or mammatocumulus), and are probably on the base of a cumulonimbus or storm cloud. Mamma occur when the upper parts of the cloud radiate heat into the atmosphere, cool and sink. If the sinking air is relatively warm and humid, the water vapour it contains will condense into cloud droplets as it mixes with colder, drier air beneath the cloud.

The process is an upside-down version of the way cumulus clouds form – the air associated with these warms at ground level and rises, its water vapour condensing to form clouds. Mamma air in the troposphere cools and sinks to form the clouds. Mamma attached to a cumulonimbus are associated with severe weather conditions, and aviators are strongly advised to avoid them.

A good summary of this phenomenon can be found in Gavin Pretor-Pinney’s The Cloudspotter’s Guide (Sceptre, 2006).

Ed Hutchinson

Cambridge, UK

Cloud cover

A group of cumulus clouds was passing over my house in Tuscany, Italy, on a September afternoon lit by the low sun at about 5 pm. A clear shadow of the lower clouds appeared on the undersurface of the upper clouds (see below). Given that the light came from above both sets of clouds, how was the shadow projected on the higher ones?

Alessandro Saragosa

Terranuova, Italy

We received lots of theories for this phenomenon, but no single conclusive idea – Ed.

I took this photograph (opposite) in August at about 5 pm. The sun did not set for another 3 or 4 hours. Like your correspondent, I was mystified by the apparent shadow cast above, rather than below the cloud. This puzzlement was compounded by the rays of sunlight visible against the shadow.

Jeanette Stafford

Glasgow, UK

The original photograph sent by the questioner contains most of the clues to what is going on here.

There are three layers of cloud in the picture: an upper broken stratus and two lower cumulus layers. The sunlight, which seems to be coming from the lower middle of the picture, is falling onto all the cloud layers, including a small part of the lowest cumulus cloud. This can be seen by the short bright edge on this cloud. The light is strongly reflected and scattered, illuminating a large area of the underside of the middle layer, as the picture shows. This cloud, in turn, casts a shadow on the upper stratus cloud.

Chris Daniel

Kingston upon Thames, Surrey, UK

The sun is at a low angle, close to sunset, so it is illuminating the clouds from underneath. Shadows from the cumulus are being cast upward, resulting in this unusual display of light and shadow.

Rachel Vis

Melbourne, Australia

The shadow is not being projected onto the higher clouds, but rather onto the air between the cumulus clouds and the camera. I have seen similar shadows just before dusk caused by planes, which showed up as dark streaks. The scattered light makes the air appear to glow, except in the areas where it is being shaded by the clouds.

Bob Mitchell

Niceville, Florida, US

Mountain headgear

I’ve seen mountain-top clouds similar to the one in this astounding image of a sombrero-like cloud atop Mount Fuji. What causes them? Do they only occur above volcanoes or do they occur above any mountain of suitable height?

Kevin Enright

Ironbridge, Shropshire, UK

This is a striking example of the cloud species known as altocumulus lenticularis (known in English as lenticular altocumulus – see also page 78). Such cloud forms when a stable, humid layer is forced to rise above the level at which condensation usually occurs, normally as part of wave motion. The uplift occurs above or downwind of an obstacle, and is certainly not restricted to volcanoes.

Depending on the exact atmospheric conditions, long trains of waves and clouds may be produced, and wave clouds have been observed far from any obvious source of motion. Such clouds tend to remain stationary as long as conditions, including wind strength and direction, remain constant. It is not uncommon for a series of humid layers to be affected, giving rise to a vertical stack of lenticular clouds separated by clear air, known to meteorologists as pile d’assiettes (‘pile of plates’).

Just as the air is forced to ascend and cool, producing condensation, so as the air descends at the rear of the wave it warms and any cloud dissipates. Close examination (with binoculars) will often reveal how the cloud is forming on the upwind side and dispersing at the trailing edge.

This cloud species is related to a similar cloud known as pileus, where an ephemeral cap of cloud forms as air is forced upwards above an actively rising cloud tower. In this case, the convective cloud cell often breaks through the pileus, producing a collar of cloud that is normally entrained into the rising column.

Storm Dunlop

Chichester, West Sussex, UK

This cloud type is a very well known phenomenon to glider pilots worldwide. The upper winds are blowing away from the camera towards Mount Fuji, probably at speeds in excess of 100 km/h at the mountain’s peak. As the wind strikes the slopes of the mountain, it is forced to rise and becomes colder and less dense, and the moisture in the airstream condenses out, creating what is called a cap cloud over the peak.

Immediately downwind of the peak – the other side of the peak in the photo – the airstream, through a combination of temperature and stability characteristics, spills down the mountainside and then rebounds upwards again before descending even further downwind. With the right atmospheric conditions, this series of stationary vertical oscillations, or standing waves, can continue for 100 kilometres or more downwind.

The top of each standing wave is often marked by what is called a lenticular cloud a few kilometres behind the mountain crest. The lenticular cloud appears stationary despite the very high wind speeds through it.

Massive standing-wave systems and their accompanying lenticular clouds are found above mountain systems all over the world, including Europe, the Andes, California and New Zealand, where the altitude record for a glider was set at 15,000 metres.

A miniature version of a standing wave can often be seen when a shallow flow of fast-moving water in a stream or gutter rides over a submerged object. This often creates two or three standing waves in the water downstream from the submerged object.

Max Hedt

Horsham, Victoria, Australia

What a beautiful lennie! As any glider pilot will tell you, this is a lenticular cloud formed in a standing wave. When flying a glider in such a wave, the ride is incredibly smooth, and if the wind speed is higher than the glider’s stall speed then you can head directly into wind and seemingly be suspended in space with just the aircraft instruments indicating that the glider is actually flying. Magic!

Mike Debney

Melton South, Victoria, Australia

Genius of the lamps

On a September evening in 2003, my husband took this photograph (see below) outside our house in Cambridge looking east. The clouds seem to have come out of Aladdin’s lamp. Can someone explain how such a remarkable pattern can be formed, or is the genie’s tale true?

Omara Williams

Cambridge, UK

Within a few days of the original photograph of the ‘Aladdin’s clouds’ being published, we received three more photographs taken of the same five clouds on the same day from different locations in eastern England (see the following two pages). One of them [A] was taken by Rowan Moore at Dunchurch, near Rugby, 100 kilometres to the west of Cambridge; one [B] was taken by Martin Williams at Holme, near Peterborough, 40 kilometres northwest of Cambridge; and the third photograph [C] by Clive Semmens in Ely, 25 kilometres north of Cambridge. The final picture [D] is of a larger group of similarly shaped clouds photographed from Nottingham, in central England, by Sean May on a different date. The explanation for these peculiar clouds has been provided by another reader… – Ed.

These strikingly shaped clouds are not rising and trailing particles below them, as their appearance in the photographs suggest. They are actually altocumulus clouds from which precipitation is falling.

Such trails of water droplets or ice particles are called virgae (or sometimes fallstreaks) and, by definition, do not reach the ground. Precipitation that does reach the Earth’s surface is known technically as praecipitatio.

Virgae are produced from what are known as heads. These can be distinct, rounded clumps of cloud like the ones that are shown here, ragged tufts, or extremely small patches of cloud that are difficult to distinguish from the tops of the virgae themselves. They occur at all cloud levels, and ordinary cirrus clouds – the thin wispy clouds that are popularly called mares’ tails – essentially consist solely of virgae.

Virgae often show distinct bends like the ones clearly visible in the photographs. These bends often occur where the falling ice particles melt into water droplets.

The ice crystals fall almost vertically. But as the water droplets start to evaporate, they become smaller and so fall more slowly, leaving them trailing behind the cloud above.

In other cases, the bend may indicate a region of wind shear, where the strength or direction of the wind changes. On very rare occasions, where the wind is stronger at a lower level, virgae have even been observed in front of the head that generated them.

Storm Dunlop

Chichester, West Sussex, UK

Clear flight path

This photograph was taken near Maldon, Essex, in the UK, looking directly overhead. It appears to show the result of an aircraft flying through thin cloud and dispersing it along its flight path. If an aircraft was responsible it had long since passed when the picture was taken. Is this a common sight and what mix of conditions is required to produce the effect?

Neil Sinclair

Chelmsford, Essex, UK

This is a relatively common occurrence, known as a dissipation trail or distrail. Depending on the exact circumstances, one of three mechanisms may be involved. First, the heat from the aircraft’s engines may be sufficient to evaporate the cloud droplets. Second, the wake vortices shed by the wings may mix drier air into the cloud, lowering the relative humidity and again causing droplets to evaporate. Finally, the exhaust may introduce glaciation nuclei into the cloud. These are particles around which ice crystals form, causing freezing to occur. The crystals then fall out of the cloud. This is a very common mechanism, but the photograph shows no sign of falling trails of ice, which are known as virgae.

The first mechanism seems to be rare and is not accepted by some authorities, so the vortex explanation is probably the most likely in this case.

Storm Dunlop

Chichester, West Sussex, UK

Misty morn

While camping in the desert north of Coober Pedy, South Australia, in July 2007, my son and I were privileged to witness a white rainbow at daybreak, similar to the one shown here. The landscape was covered in mist and the white rainbow’s arc seemed to grow as the sun came up, though it faded away as the mist evaporated. I’ve asked old swagmen, indigenous locals and several lecturers at two universities, but no one has ever witnessed the phenomenon. What caused it?

Fred Richardson

Alice Springs, Northern Territory, Australia

This was a fogbow, sometimes known as a cloudbow or mistbow. Like a primary rainbow, it is centred on the point opposite the sun and has an angular radius of approximately 42 degrees. It is caused by the same mechanism: reflection and refraction of sunlight by water droplets. In this case the droplets are unusually small – less than 50 micrometres across – allowing diffraction to spread the bands of colours so that they overlap and appear white.

Occasionally, fogbows will show a bluish tinge to the inner edge and a reddish one on the outer. From some viewpoints, such as an aircraft, a fogbow can appear as an almost complete circle.

Storm Dunlop

Chichester, West Sussex, UK

A fogbow is a frustrated rainbow, formed in essentially the same way. Sunlight destined to create rainbows and fogbows is refracted twice – once as it enters a water drop and again as it leaves. While inside the water drop between the two refractions, the light bounces off the inside back surface, sending it heading back towards the sun. This is why rainbows and fogbows are seen when the sun is behind the observer. The path of blue light is bent more than red by the droplet, usually causing sunlight to be dispersed into the colours of the visible spectrum with blue at the bottom of a rainbow and red at the top.

Rainbows appear white when water droplets are less than 100 micrometres across – small enough for diffraction to dominate over refraction. Each water drop forms its own diffraction pattern – bands of alternate constructive and destructive interference – for each colour: the smaller the drop, the broader the bands. When the drops are small enough, these bands become so broad that all the colours overlap, essentially mixing them all together again to make white.

Beautiful images can be found at

Mike Follows

Willenhall, West Midlands, UK

Hair-raising event

Walking along the breakwater at Berwick-upon-Tweed in north-east England, my granddaughter and her mother noticed their hair was standing on end. It started to rain soon afterwards, but there was no thunder or lightning that day. What was happening?

Richard Turner

Harrogate, North Yorkshire, UK

We have answered this question before in an earlier book, but this time we are able to use the startling photograph the questioner supplied originally – Ed.

From one of my physics textbooks I recall a hair-raising picture of a woman standing on an exposed viewing platform at Sequoia National Park in California. She was in grave danger. Lightning struck only minutes after she left, killing one person and injuring seven others (Fundamentals of Physics, 6th Edition, by David Halliday, Robert Resnick and Jearl Walker, published by John Wiley and Sons). It’s likely that similar conditions were abroad on the day your photo was taken.

Most lightning clouds carry a negative charge at their base. Anything close to the cloud would feel the effect of electrostatic forces: electrons in a person’s hair would be repelled downwards, leaving the ends of the hair positively charged. The positive hair