Three main ingredients are necessary for a snowstorm. First, the space between the clouds and the surface of Earth must have below-freezing temperatures. Second, moisture must be present in order to form the clouds that eventually create snow. Snowstorms get much of their moisture when air blows over a large body of water. Finally, something must lift this moist air high enough to form moisture-rich clouds. One of the common sources of lift occurs at the boundary of warm and cold air masses. When these conflicting air masses (called a front) collide, moist air is forced into the atmosphere. Wind that blows over a mountain range is also another source of lift. Snow itself consists of tiny ice crystals that bond with one another as they fall towards the ground. Snow usually passes through a band of warm air as it approaches the ground. Assuming this band of air is below freezing (32°F), snow is capable of making its entire journey without melting.
Snowstorms present meteorologists with some of the most difficult forecasting dilemmas. This is because snow generally falls heaviest within a narrow band. Furthermore, the boundary line between snow and rain can change considerably with small changes in temperatures. Because of this, an all-snow forecast can quickly turn to mostly rain or vice versa.
Many people go by the rule that one inch of water equals 10 inches of snow. While this is approximately true of snow that occurs at temperatures close to the freezing point (or snow that is accompanied by strong winds), it is not always true. Some snows can have as little as 0.1 inch of water for every 10 inches of snow, while others may have as much as 4 inches of water for every 10 inches of snow. Crystal structure, wind speed, and temperature are some of the factors that determine the water content of snow. In the United States, most water to snow ratios are between 0.04 and 0.10. Snow that contains a high water content results in a heavy and dense snow. This is certainly true of snow that falls near 32°F. At temperatures around 15°F, snow is potentially its fluffiest and lightest. When the temperature drops to 0°F or less, snow crystals become smaller, thereby increasing its density level.
The crystalline structure of snow makes it an excellent reflector of light. What little light is absorbed by snow crystals is distributed evenly over the entire spectrum of light, thereby giving it its white appearance.
When snowstorms occur, they can be accompanied by heavy accumulations, strong winds, extreme cold, ice, sleet, and freezing rain. These factors are what meteorologists use to classify a storm. Snowstorm classifications include flurries (snowfall that results in almost no accumulation), showers (varying intensity and light accumulation), squalls (brief, intense showers accompanied by strong winds), blowing snow (wind-driven snow that lessens visibility and causes significant drifting), and blizzard (heavy snow accompanied by heavy winds and extremely low visibility). Sleet (frozen ice pellets) and freezing rain (rain that freezes when it makes contact with below-freezing surfaces) often mix with snow, especially if temperatures are close to freezing.
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