Snowflakes are one of nature’s most fragile things, but just look at what they can do when they stick together.
Snowstorms
While light snowfalls occur often, these are typically mostly a concern for travel (e.g. car, plane, or school bus). These events do not usually disrupt whole communities and created lengthy travel delays. Once snowfall totals reach ( or are expected to reach) about four inches (10 cm) in 12 hours or six inches (15 cm) in 24 hours, heavy snow terminology comes into play. Criteria for heavy snow can be locally adjusted for mountain and lake effect areas where these snowfall amounts are more commonly observed. Similar limits for reaching “heavy snow” exist in countries around the world.
Heavy snow most often occurs along the track traced by traveling cyclones. This is the “cold side” of the cyclone. Snowfall amounts depend upon the track of the cyclone, its speed of movement, and the availability of moist air. In mountainous areas, the cyclone may not be as evident on surface weather maps, but is still present in weather patterns in the middle and upper troposphere.
Typically, the band of heaviest snow is long and relatively narrow. Often, not far from the band of heaviest snow, precipitation transitions to sleet, freezing rain and/or rain. In larger counties or even over large cities, it is not unusual for heavy snow to fall over northwestern areas and rain to fall over southeastern areas.
Ground temperatures leading up to, and during, the snow event and the rate at which snow falls are key to whether the snow provides picturesque scenes or whether roadways become snow-covered and slippery. Often secondary and side roads become snow-covered most quickly with main highways mostly free of snow pack. This is because the weight of many cars moving over snow compresses and warms the snow, partially melting it. Also, main roads are plowed and treated with chemicals earlier than side roads. Airports are often the first to feel the pinch, with fewer flights allowed to land and take off due to spacing and ground control factors. If the snow falls too quickly or the winds are too strong, some runways may be shut down, in the worst case; airports may cancel large numbers of flights or even shut down. While snowstorms can create incredible beauty, including sculpting of the snow by winds, they can be deadly, economically disastrous and socially disruptive.
