3rd March 2006

Snow

Source: Excerpt from The Book " Weather "

Distribution: Common in high and middle latitudes but may occur near the equator at high altitudes. Cause: Ice crystals bonding in cloud and then falling through cold air. Associated Weather: Low temperatures and build-up of snow at ground level. Hazard Warning: Heavy falls combined with high winds may lead to blizzards, whiteouts, and avalanches. A landscape draped in a thick mantle of fresh snow is one of nature's most magnificent sights. Snow is common during the winter months in Europe and North America and is a permanent feature of many mountaintops throughout the world. Mount Kilimanjaro, in Tanzania, maintains a permanent cap of snow despite being only 3 degrees south of the equator. Snow begins as ice crystals that form a cloud when water vapor freezes around minute solid particles in the middle and upper levels of the atmosphere, where the temperatures are well below 32° F (0° C). The individual ice crystals gradually bond, forming snowflakes. Once the snowflakes are heavy enough, they fall to the ground. Ice crystals form in a vast array of shapes depending on the temperature and humidity of the surrounding air mass. With the invention of the microscope, the beauty and diversity of ice crystals became apparent for the first time. An American farmer, William Bentley (1865-1931), photographed thousands of magnified ice crystals, and noted that, although there were identifiable crystal systems, no two crystals were identical. In order to study individual crystals, Bentley had to catch the flakes on a velvet-covered tray, tease the crystals apart with a probe, and smooth them out with a feather. The Temperature of Snow Often snow that falls from a cloud melts as it descends, and reaches the ground as rain. However, the melting process extracts latent heat from the surrounding air, causing the air temperature to cool and making it increasingly likely that subsequent snow will reach the ground. Interestingly, the ideal conditions for snow are temperatures close to and just below zero, rather than colder temperatures. This is because the warmer the snow, the more moisture it will contain, and hence the bigger the flakes will be; and because a temperature close to 32° F (0° C) will allow snow to melt, refreeze, and combine in larger flakes. As a result, very slight changes in temperature can mean the difference between snow or rain, making accurate forecasts difficult. Snow can settle on the ground in a variety of forms, depending on wind, temperature, and humidity. Air temperatures well below freezing produce small, powdery flakes that provide ideal conditions for skiing. Snowflakes that form in temperatures closer to 32° F (0° C) are larger and wetter and tend to stick to surfaces. Strong winds may pile up snow in hollows and against houses, in what are known as snowdrifts. Once snow has settled, it may melt and refreeze, becoming harder and more compacted in the process. In mountainous areas, a build-up of snow can cause avalanches, which may sweep down steep slopes, destroying everything in their path. This is often the result of new falls of loose, powdery snow settling on a hard base formed by earlier falls. More common, but just as dangerous, are blizzards. These are caused by a combination of heavy snow, low temperatures, and strong winds, and can bring cities to a standstill. An associated phenomenon is a whiteout, when heavy snow and low cloud render the ground and the sky indistinguishable, making navigation impossible. In countries where these conditions occur, blizzard warnings are among the most important weather forecasts issued. Accurate forecasts can reduce fatalities by ensuring people are indoors by the time a blizzard occurs, and by alerting emergency services before conditions deteriorate.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.