Freeze questions: What is snow?

We answer the questions you never thought to ask

(PA)

Snow forms at temperatures below 0C, when water freezes by forming crystals around "seeds" of particulate matter such as dust. These crystals come in many different forms, from needles and hexagonal prisms to the beautiful plates and branching, star-like forms we call snowflakes.

Snowflakes are composed of individual ice crystals that have collided within a cloud and frozen together, producing an infinite variety of forms.

At very low temperatures, snowflakes remain in a powdery "dry" form, such as those that fell at the beginning of the snow storm in South-east England on Monday. At higher temperatures, closer to 0C, the snowflakes stick together to become larger and "wetter". This type of snow can easily turn to rain and sleet. Wet snow is more common in Britain than the powdered variety, which famously interfered with the hydraulic systems of British Rail trains when it fell in February 1991, and will forever after been known as the "wrong kind of snow".

Steve Connor, Science Editor

Will there be a surge in births in nine months?

Trains aren't running, offices abandoned. What to do but snuggle under the duvet and...

It is widely believed that any natural event that keeps people confined to their homes results in an immediate boost to the conception rate – whether blizzards, earthquakes, or transport strikes. Nine months after a blackout that plunged New York into darkness for 10 hours in 1965, it was reported that several hospitals saw a baby boom. Closer analysis revealed that the number of births was actually within the normal monthly variation. But the myth has persisted. As a New York Times writer observed at the time: "It is evidently pleasing to many people to fantasise that when people are trapped by some immobilising event... most will turn to copulation."

Jeremy Laurance, Health Editor

What caused this week's blizzards?

A cold mass of continental air arrived in the South-east from the Arctic via western Russia. It picked up moisture as it passed over the relatively warm North Sea, which at this time of the year is about 5C. Once it hit colder land, the moisture stored within clouds that had formed in the air mass fell as snow on Sunday night and Monday.

Meanwhile, a low pressure area developed to the south of the British Isles, which moved north. This rather warmer air, just above 0C, mingled with the cold continental air and added extra moisture to the mix as it travelled north. The result was that even more snow fell as it crossed the country.

Steve Connor, Science Editor

What's the best way to clear a drive of ice?

The easiest way is to brush away the snow before it can melt and refreeze into ice, which makes the clearing process so much more difficult. Adding salt lowers the freezing point of water and so aids the break up of ice, although this too can refreeze if the temperature falls well below 0C.

Spreading gritty sand over ice helps to prevent slipping. The grit used on roads is rock salt mined from the ground and crushed to form light brown granules, which in turn is crushed by traffic, spreading it far and wide.

Steve Connor, Science Editor

Is it safer to drive or cycle in the snow?

Generally speaking, four wheels are more stable than two. However, a car sliding on ice is a danger to others as well as the driver, whereas a sliding bike is only really a danger to the rider. Being heavier, a car is more likely than a bike to get out of control on ice – unless the car has snow tyres. A cyclist also has a better awareness of the road conditions than a driver so is more likely to be cautious when riding in snow, and is more likely to be able to avoid icy patches. So on balance, it is probably safer to ride a bike than a car, providing you take it easy.

Steve Connor, Science Editor

How is snowfall measured?

The Met Office has 193 weather stations across the country, where it measures both how much snow is laying on the ground and how much has fallen from the sky.

The former is measured by man or machine, using a ruler to determine how much snow is underfoot. The latter is done in the same way as rainfall is recorded. Snow is collected, as it falls, into a copper container and then melted. The Met Office uses the calculation that 1mm of water is equivalent to 1cm of snow.

Mark Hughes

Can we eat snow?

The closest most people will have come to a snow dish is a light dusting of icing sugar on a dessert, but anyone with a handful of fresh snowflakes can whip up a frozen marvel in seconds.

Naturally, these snow delicacies tend to be at the icy end of gastronomy: snow ice cream, snow sorbet and so on. And, as they usually contain large amounts of sugar and cream, are not suitable for those on a diet.

First, though, we should consider how safe snow is to eat. As frozen precipitation, it's surely as (un)healthy as rainwater – and most of us wouldn't scoop up handfuls of water from puddles and make soup from it.

Yesterday it was hard to get an official answer to the question of whether we can eat snow. A press officer at the Drinking Water Inspectorate replied: "We only deal with the water companies and mains water. Try the Environment Agency."

They said: "We don't seem to be regulating snow. You should call the Health Protection Agency."

Their spokesman's response was: "I don't think we could answer it. I suppose you could call the Department of Health...," who said: "Thank you for calling the Department of Health newsdesk. You call is being placed in a queue..." (Later, a spokesman sent a statement saying: "If the snow is fresh, untrampled or untampered then there is little risk of contamination and so it can be consumed. If it is not virgin snow then it is advisable to boil it before drinking as a source of water.")

Luckily, some of the finest minds on the internet have already applied themselves to this question; and the answer isn't necessarily good news. A Los Angeles Times report warned: "Snowflakes drifting out of the sky may have a surprise inside: bacteria."

The Drinking Water Inspectorate advises people not to drink untreated water, warning it may be contaminated with bacteria, protozoa, parasites or viruses. "Many of these are harmless, but some may cause serious illness or even death," said a spokesman.

And so to a recipe:

Chocolate Snow Ice Cream

Ingredients:

Big bowl of snow

One cup sugar

One cup chocolate milk

Instructions:

Mix together

But if you can't be bothered making your own snow dish, the BBC Saturday Kitchen chef Paul Merrett is serving oysters in virgin Richmond Park snow at his gastro-pub, The Victoria, in Sheen, west London.

Martin Hickman

Does it make us happy?

Does the dawn sun? The smell of bacon frying? Crisp clean sheets? Of course. A soft blanket smothering the noisy, confused world in silent, virginal white? Only a curmudgeon could be left unmoved.

Psychologists say it all depends on how you see it. An appreciation of beauty enhances wellbeing, and this can be measured (via the stress hormone cortisol, levels of which are lowered). But if you peer through the curtains and think only of the plane you have to catch, the car that won't start or the school that your children will not be going to, it will have the opposite effect.

"Go for a walk in the park in the snow and it will de-stress you," says the aptly named Ilona Boniwell, a specialist in the psychology of happiness and wellbeing at the University of East London. The transformation of the world should be uplifting, she says. Provided you see it as a gift, not a problem.

Jeremy Laurance, Health Editor

Should you have a hot toddy in the cold?

The image of a St Bernard reviving avalanche victims with a slug of brandy from the barrel round its neck has entered popular myth as the ideal way to deal with the menacing cold. St Bernards no longer patrol the slopes of the Alps, and probably never carried brandy anyway, which is about the worst medicine you can give to someone who is lying in the snow. Alcohol dilates the peripheral blood vessels, causing blood to rush to the skin and the cheeks to flush, releasing a comforting warm glow. In doing so, however, it causes the body temperature to fall – the last thing you need while exposed to the elements. So save the hot toddy till you are back indoors and warming your toes by the fire.

Jeremy Laurance, Health Editor

How long will the snowy weather last?

At least until the end of the week, with at least as much snow as fell on Monday expected again. Barry Grommet of the Met Office warns that southern and central England is "not out of the woods yet", with "significant amounts" of snow still to come. Most of the country will experience freezing conditions tonight, and the Met Office has issued a severe weather warning for both Thursday and Friday, with temperatures dipping as low as minus 5C. Up to 10 inches of snow could fall in some parts of the country on Friday alone. Rain, sleet and strong winds will sweep across central and southern England in particular, only letting up slightly at the weekend.

Amol Rajan

What's the safest way to drive?

If a road has not been gritted, avoid driving in the tracks of other vehicles, where compressed snow will provide less grip than the relatively undisturbed snow around it. Visibility, which is impaired in snowy conditions, can be improved by wearing sunglasses, which reduce the glare of a low winter sun. Windscreen washer bottles need the right balance between water and solution, and should be checked to make sure they're not frozen. The best way to avoid skidding is to keep speeds low and apply the accelerator and brakes gently. Once in a skid, it's best to lift off the accelerator and steer in the direction of the skid.

Amol Rajan

Is it illegal to throw a snowball at a stranger?

Some police forces have announced a "crackdown" on children throwing snowballs. Hertfordshire police said they might arrest or fine "anyone who is causing antisocial behaviour or who is acting irresponsibly in these current conditions".

Chief Inspector Nigel Brown said: "We understand people want to have fun in the snow but would ask members of the public to think about the consequences of their actions."

Generally, throwing snowballs does not qualify as assault because those being hit have usually consented. But if they have not, and if they are placed in fear of injury or indeed are injured, throwing a snowball could be construed as actual bodily harm, grievous bodily harm, or worse.

Amol Rajan

How can we help garden birds?

In really harsh weather such as we have been experiencing this week, garden birds – and indeed all wild birds – definitely need help or they will not survive. They face a double problem in periods of heavy snow: the need to maintain their energy levels while having difficulty finding food. In continuous cold, small birds lose body heat very quickly and expend a lot of energy trying to maintain it, yet the fuel to do this is often unavailable as the ground is covered, it is too hard to dig out worms and insects, many autumn berries are long gone and water is frozen. According to the Royal Society for the Protection of Birds, seed-feeders are one of the best ways to help as modern specialised bird seeds such as sunflower hearts or nyjer seeds are very rich in nutrition. They need to be regularly filled as they will quickly run out, bird tables need to be regularly cleared of snow and water supplies need to be changed. The RSPB says that under these conditions food put out into gardens is a real lifeline which can make all the difference to wild birds' survival.

Michael McCarthy, Environment Editor

So should we stop worrying about global warming?

No. This week's snowfall is a phenomenon of weather. Global warming is a phenomenon of climate. The difference between weather and climate is not always appreciated, but it is crucial for an understanding of climate change and how it may begin to affect us. In simple terms, weather is what happens this week; climate is what happens this century. One is a short-term phenomenon, the other is a long-term one, and while over the long term we can perceive a trend, over the short term what we perceive is variability. The natural variability of the weather is enormous. In essence, it is infinite. So it is virtually impossible to deduce anything about climate or climate change from a single weather event. Even as the climate warms, some years will be cooler than others, and climate history shows us that the warming will not proceed in a steady, orderly – in scientific terms, linear – way. It will go in leaps and bounds, and there may be periods of years when the warming seems to have paused, followed by years of rapid warming. So don't look at the event. Look at the trend. And despite this week's snow, the temperature trend for Britain is remorselessly up; the average temperature of the UK has risen by a full degree Centigrade over the past 40 years.

Michael McCarthy, Environment Editor