Daily Telegraph

White Christmases are becoming thinner on the ground. The slide started in 1582 when Pope Gregory XIII cut ten days out of the calendar, thus making Christmas arrive earlier, when the weather is not quite so cold. Global climatic change continued the trend. By the time that Charles Dickens came to write his description of a romantic, snowy English Christmas in “The Pickwick Papers” in 1837, the three hundred year-long “mini-ice age” that had gripped Europe was coming to an end.

In those days, London experienced a White Christmas about one year in every four. Now the odds now are more like one year in every ten. The odds improve as you go North, which is probably why Queen Victoria and Prince Albert were so keen to have a castle at Balmoral in Scotland to retreat to at Christmas.

Moving to more Northerly latitudes is one way to increase the chance of a “White Christmas”. The phrase implies to me a crisp layer of snow covering the ground on Christmas morning, but modern bookmakers have a different definition. To them, a “White Christmas” is one where as little as a single flake of snow has fallen. The bookies’ definition immediately doubles the chance of a White Christmas, but even then London still only experiences a bookies’ White Christmas for a paltry one year in five (William Hill offer odds of 6:1 against at the time of writing). “Can you beat the bookies with science? Is there a scientific way, however crazy, to increase the chance of a White Christmas?” When Roger Highfield, the Telegraph’s science editor (and no stranger to the science of Christmas), posed these questions, I could think of several answers. If the snow won’t come to the ground, for example, take the ground to the snow.

You could go to the top of the highest building you can find, where the air is colder and the chance of a snowflake surviving without melting as it falls is correspondingly greater. You could even go up in a balloon, and have Christmas in the clouds. It would have to be a helium balloon, though. A hot air balloon would melt the very snow that you came for.

At ground level, one could use a snow machine. Those that are used in ski resorts produce a compressed blast of warm, moist air that cools rapidly to below freezing as it expands. The air is loaded with solid particles that act as “nuclei” on which some of the water vapour condenses directly to ice. The water vapour that does not get picked up by the particles forms droplets of liquid water, which gradually evaporate. The vapour is collected by the tiny ice-covered dust particles, which grow into snow crystals and sometimes stick together to become snowflakes.

A snow machine would probably be regarded as cheating. It would be far better to use Nature’s snow machine, which operates to especially dramatic effect over the Great Lakes of the Eastern United States. Here the cold, dry winter winds from the Arctic blow across the lake surfaces while those surfaces are still warm from the summer. The wind collects the excess moisture, and the moist, warm air mass rises rapidly as it hits the lee shore, cooling and filling with snow which it dumps in prodigious amounts on cities such as Buffalo.

To encourage a White Christmas in London, all we need to do is to reproduce these conditions on its outskirts. England does not have large lakes, but these could be manufactured by flooding the Norfolk broads and the fens near Cambridge. Giant heaters could be placed in the resultant lakes to enhance their moisturising effect on the wind. We could then sit back and let the bitter Northerly winds of winter do the rest. The rising mass of warm, air moist air could be encouraged to produce the maximum amount of snow as it cools by adding the right sort of particles to act as nuclei. Some bacteria are known to be particularly efficient nucleators, but their use would probably be forbidden under health and safety regulations. In any case, there is no need to go that far, because the well-rotted manure that farmers spray on their field in winter contains billions of particles of the right sort. Instead spraying manure from tractors, farmers could drop it from aeroplanes above the clouds onto their sodden fields, promoting snow as well as fertilizing the ground. If the smell was thought to be too awful, other aeroplanes could follow, dropping perfume to produce perfumed snow. The London planning regulations could also be changed to encourage the building of skyscrapers along the Northern boundary of Greater London. A line of such skyscrapers would behave as a mountain, pushing the moist air masses rapidly upwards to form and discharge a load of snow.

Science tells us that the resultant snow will only reach ground level if the wet-bulb temperature of the air – i.e. the temperature measured by wrapping the bulb of the thermometer in a bit of damp cloth and swinging the thermometer around – remains below zero all the way to the ground. One way to achieve this would be to take a bit of the right weather from somewhere else and import it. We could, for example, set up huge parabolic mirrors in space to focus the image of a cold place, such as the top of Ben Nevis, onto the appropriate area of London. But there is another way, now being seriously studied by U.S. meteorologist Ross Hoffman.

The idea comes from chaos theory, which says that a tiny effect in one part of the world can magnify to create a huge effect somewhere else. In the famous words of Edward Lorenz, the flap of a butterfly’s wing in Brazil could set off a tornado in Texas. Hoffman’s idea is to flap the butterfly’s wing deliberately. In other words, by controlling the atmospheric conditions in one part of the globe, it should be possible to influence weather patterns in other regions. According to his computer calculations, the control would have to be on a fairly massive scale, but it could be achieved by having satellites with steerable mirrors to reflect sunlight onto required spots on the planet. Perhaps the sun focused on Buffalo in mid-winter would produce snowy conditions in London, with benefit to both parties, and also to the computer companies which would have to produce the hardware for the massive amount of computation involved for such a relatively short-term effect. If all of these ideas fail then global warming, which has been blamed for the decreasing frequency of White Christmases, might in the end come to their rescue. Warm air masses moving from equatorial regions may eventually produce snowy conditions near the poles more frequently. The climate flip scenario, where the Gulf stream is switched off by global warming, could also cool the U.K. climate. With the aid of global warming, Britain could become a tourist destination for cooling down from the sultry conditions of the rest of the world, with more White Christmases than Charles Dickens ever dreamed of.

Scientific facts about snow

Snowflakes emit a high-pitched scream when they land on water. The noise comes from trapped air bubbles vibrating spontaneously at 200,000 times a second. The shape of snow crystals tells us about the atmospheric conditions under which they were formed. The great Japanese physicist Ukichiro Nakaya, who discovered the relationship in the 1930s, called snowflakes “letters from the sky”. Later scientists have discovered that these “letters” also contain a record of atmospheric pollution.

The notion that no two snowflakes are the same came from a book of 2500 photographs published by an English farmer called Bentley in 1931. In 1988, though, the U.S. meteorologist Nancy Knight discovered two snowflakes that appeared to be exactly the same shape.

© This article is copyright Len Fisher. Please email Len Fisher to seek permission to reproduce part or all of the above article.

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