A plow truck works to keep roads clear during a snow storm in southern Wisconsin.

When Disaster Strikes: Blizzards
(Steven Otfinoski)

WHAT is snow?

Snow is a well known hazard to those living in mountainous areas and regions north of 35 degrees latitude in the Northern Hemisphere. Automobile accidents, transportation delays, damage to buildings, and dangerous walkways are often attributed to snow during the winter months. Although slippery surfaces are often the primary cause of such problems, reduced visibility is also responsible for many transportation accidents. Snow is often associated with low temperatures, which can be life-threatening to humans and other organisms. Snow can be relatively warm, causing wet and slushy conditions, or cold, creating dry and powdery conditions. The latter leads to blizzards and drifting when mixed with high winds. Blizzards can quickly reduce visibility on roadways, while their high winds and moisture combine to quickly reduce body temperatures and cause frostbite on exposed skin. Drifting can block roadways, airport runways, and even bury buildings. In many mountainous regions, avalanches are a common hazard when large accumulations of snow become unstable on steep slopes. When large stores of mountain snow begin to melt in the spring, floods often become an imminent hazard to people living in the valleys below, especially in the presence of spring thunderstorms.

WHERE does snow occur?

Snow falls somewhere at every latitude on the planet. In tropical and subtropical latitudes, it is limited to high elevations. Here, snow poses the same threats as it does at low elevations in polar latitudes. Frozen, blowing precipitation mixed with cold temperatures and poor visibility is hazardous anywhere, to humans and other organisms alike. In general, the higher the latitude, the greater amount of snow that is received. Exceptions to this are mountainous regions that interact with the colder upper atmosphere, and cold coastal areas that receive more moisture than inland locations.

Snow is common across most of North America, Europe, and Asia. Between the months of November and April, latitudes north of 40 degrees N receive an average of 25 inches (64 cm) of snow or greater. At high elevations in mountain regions, snow can begin to fall as early as August and remain through June due to cool temperatures throughout the year. Even areas south of 35 degrees N can have snow accumulation and several nights below freezing. The total amount of snow received at any location is just as dependent on the topography (the shape of the land) and the availability of moisture sources (such as oceans and lakes) as it is on temperature. Topography has a pronounced effect on snowfall because mountains ranges and hills act as barriers forcing air to rise and cool. As air cools, moisture is forced out, usually resulting in much greater snow accumulations on the windward side of mountain ranges. For the atmosphere to hold enough moisture to produce snowfall, a source of evaporation is needed. Oceans and large unfrozen lakes can provide ample moisture.

In the year 2000, winter storms caused approximately $1 billion of damage in the United States.

WHEN does snow occur?

Snow is a threat to most of North America during the northern hemisphere's winter, which begins astronomically on December 21 and ends March 21. However, many areas within North America receive substantial snow accumulations beginning in October and lasting into April. This is especially the case at high elevations in mountain ranges. At the beginning and end of the winter season, snow becomes warmer, giving it a greater tendency to melt on contact or stick to the surface. This greatly reduces the threat of drifting. However, if below freezing temperatures follow, melted snow may refreeze, causing roadways and walkways to become covered with ice.

HOW do we cope with snow?

The most important aspect of mitigating snow hazards is forecasting and issuing warnings, for the purpose of community preparation. Communities can prepare for winter storms by stocking sand and salt to improve road conditions, advising people to use caution when leaving their homes, and advising people to stock food before a storm in case they are not be able to leave their homes during a storm. In the United States, the National Weather Service issues winter storm warnings, blizzard warnings, heavy snow warnings, and other winter weather advisories. Mitigating snow-related transportation hazards in North America involves the use of snowplow trucks to clear roadways, de-icing agents to reduce the accumulation of snow and ice on road surfaces, and spreading sand to improve traction.

WHO has more information on snow and ice?

• Cyberspace Snow and Avalanche Center
  www.csac.org

• US National Severe Storms Laboratory Winter Weather Safety
  www.nssl.noaa.gov/edu/safety/winterweather.html
  
  
• US National Weather Service, Bismarck, N.D. Winter Weather Page
  www.crh.noaa.gov/bis/winter.php