Snowstorms usually occur between November and April in the northern hemisphere, but can appear as early as October or as late as May. It seems illogical, but spring snowstorms may actually produce more snow than winter ones, because of the extra energy brought to them by moist tropical air masses. The northern and southern hemispheres have opposite seasons of course, and opposite stormy months.
There must be cold air, both in the clouds and near the ground, for snow. Air full of moisture is also necessary, to form the snow. There must also be lift, provided when warm air is lifted over a mass of cold air, or when air rises as it meets a mountain range.
The place where warm and cold air masses meet is called a weather front. It may be a warm front, where advancing warm air slides up and over a cold front. An advancing warm front may bring sheets of stratus clouds that drop snow. A cold front, on the other hand, is a relatively abrupt front where cold air runs under a warm front that is pushed back and up. Cold fronts are typically characterized by strong shifting gusty winds and a sharp temperature drop.
Maritime air masses form over water, and carry the moisture they picked up at sea. Continental air masses, by contrast, are relatively dry. Polar and arctic air masses are cold, of course, and tropical air, such as from the Gulf of Mexico, is warm.
In a typical snowstorm in the United States, moist maritime air comes into contact with dry continental air. Maritime air doesn’t need as much moisture to make snow as it might to produce rain, because snow, an aggregate of ice crystals, is less dense than water. According to the National Snow and Ice Data Center, 10 inches of snow can include as little as 1/10 inch of water, or as much as 4 inches. Lighter snow does blow around more easily, and more easily forms drifts, making the effects of a storm worse.
Whatever amount of moisture it contains, at the weather front the warm air rises, while the cold air goes beneath. Layers of clouds form at the intersection, and precipitation begins. If the lower air layer is cold enough, snow will fall, or rain will freeze as it falls.
Sometimes extremely cold air masses may burst southward, displacing tropical maritime air. There will be strong winds, over 35 mph, blowing snow, and a wind chill that drops the temperature. Visibility may fall to less than 1/4 mile. If these conditions continue for three hours or more in the United States, it’s officially a blizzard.
A lake effect snowstorm occurs when dry arctic or polar air passes across water, such as the Great Lakes, and picks up warm moisture which it then drops on land as snow. Lake effect snow contributes to Rochester and Buffalo, located on Lake Ontario and Lake Erie, being the snowiest large cities in the U.S. Some lake moisture may make it all the way to the Appalachian Mountains however, before it is given up to the land as snow.
The fearsome Nor’easter is a cyclonic storm that strikes the northeastern coast of North America and Canada with rain, snow and great crashing waves. It may bring Arctic cold and hurricane force wind.
Safe inside, it is wonderful to enjoy the awesome power of a snowstorm, and children love the way big storms close the schools. But faced with the difficulties rough weather brings, it’s also natural to long for spring.