Under the Microscope:Snow is one of nature's most beautiful weather phenomena, of which, unfortunately, we see relatively little in Ireland any more because of our warming world. The physics behind the formation of snow, and behind the beauty of snow crystals and snowflakes, is very interesting and I will describe some of it in this article. A fuller account can be accessed at www.snowcrystals.com, created by Dr Kenneth Libbrecht, of the California Institute of Technology.
Snow is made of transparent snow (ice) crystals formed around tiny dust particles in the atmosphere when water vapour condenses at temperatures below freezing. A snowflake can be a few snow crystals stuck together or a large agglomeration of snow crystals that form the "puffballs" that float to earth in a snowfall.
When water freezes into ice it forms crystals with six-sided (hexagonal) symmetry. Snow crystals have a sixfold symmetry and ultimately this derives from the sixfold symmetry of the crystal lattice in ice. Snow is not frozen raindrops. Sometimes raindrops do freeze as they fall to earth, but we call this sleet. Snow is formed in clouds when water vapour condenses directly into ice.
Water vapour rises into the atmosphere from earth as evaporation from oceans, rivers and lakes, as transpiration from plants and as exhalations from animals. As the air cools, the water vapour condenses. Near to the ground this will form dew, but high above the ground water vapour condenses onto particles of dust, and a cloud is a collection of a countless number of these droplets suspended in the air.
Even when the winter temperature is below freezing, snow-forming clouds still mostly contain water droplets made of super-cooled liquid (ie, liquid below freezing point). These clouds also contain water vapour that evaporates from the liquid droplets. As the clouds gets colder, some droplets freeze (starting at around minus 10 degrees) but others remain liquid. Water vapour then delicately condenses on to the ice droplets, and this is how snowflakes grow. As water vapour condenses from the air, the remaining water droplets evaporate further, causing the snowflakes to grow faster. There is a net flow of water from the liquid to the solid state.
As the snow crystals grow, the regular patterns emerge. The hexagonal prism is the most basic form of a snow crystal. The hexagonal prism has two hexagonal "basal" faces and six rectangular "prism" faces. Depending on whether the basal or prism faces grow more quickly, the hexagonal prism overall will be "plate-like" or "columnar". Very small snow crystals are mostly simple hexagonal prisms, but, as they grow, branches sprout from the corners to make more complex shapes. The eventual shape attained depends on exact temperature and humidity conditions.
Fallen snow remains on the ground until it melts. The density of fallen snow can vary widely. Commonly, new snow has a density between 5 per cent and 18 per cent of water. The water equivalent of snow is a commonly used concept and is the thickness of the layer of water the snow would melt into. Snow will settle under its own weight once on the ground and will pack down to a density of about 30 per cent of the density of water. Melting and refreezing can cause further increases in density.
The record for the largest season of snowfall is held by the Mt Baker Ski Area, Bellingham, Washington, US, which had 29 metres of snow during the 1998-99 season. The world's largest snowflakes are recorded in The Guinness Book of Records as falling in January 1887 at Fort Keogh, Montana, US. One snowflake is said to have been 38cm wide.
Snow appears white because of the large number of reflecting surfaces in snow crystals. Albedo (derived from albus, the Latin for white) is an important concept in climatology. The albedo of an object is the extent to which it reflects light, and is defined as the ratio of reflected to incident radiation. Albedos of different materials range from around 4 per cent for charcoal up to 90 per cent for fresh snow. The average albedo of the earth is around 30 per cent, and obviously albedo is an important regulator of earth temperature. Human activities such as forest clearing and farming have changed the albedo of various parts of the earth.
Snow-temperature feedback represents a classic example of the albedo effect. When a snow-covered region warms, snow melts, albedo decreases and more sunlight is absorbed to accentuate the warming. The opposite happens when snow falls, initiating a cooling cycle.
Snow blindness is a painful condition caused by the exposure of unprotected eyes to ultraviolet (UV) light reflected from snow or ice in bright sunlight. This is a particular problem in polar regions and at high altitudes (the intensity of UV radiation increases with altitude). Snow blindness is a sort of sunburn of the cornea and conjunctiva and it may not be noticed until several hours after exposure. Symptoms include bloodshot, teary, gritty-feeling eyes that swell up.
Severe snow blindness can cause permanent blindness. Proprietary wraparound sunglasses should be worn when skiing, mountaineering or trekking through snow. The Inuit carved goggles from caribou antlers to prevent snow blindness. A long thin slit, which allowed in only a small amount of light, was cut into the goggles.
Snow is a particularly welcome visitor at this time of year, but it rarely calls any more. This is a pity because the snow-covered landscape is peculiarly beautiful and children derive wonderful pleasure from building snowmen, throwing snowballs, and so on.
Best wishes to all for a happy and peaceful new year.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC (http://understandingscience.ucc.ie)