BACKGROUND: Thundersnow is a thunderstorm that has snow reaching the surface instead of rain. Usually thunder and lightning are more commonly observed in warm seasons. A severe thundersnow occurs when the snow is accompanied by hail that is at least three-fourth of an inch in diameter, or when wind speeds reach at least 50 knots.
HOW STORMS DEVELOP: Storm clouds form as moisture evaporates from the earth into the atmosphere, where the droplets congregate and jostle against each other. The air cools off rapidly with altitude. Sometimes a cold front -- the boundary between where the cold air from one thunderstorm meets the air outside the storm for example ý will force the moist air upward into the colder air. This moist air cools off and the water vapor "condenses" into liquid drops, forming clouds. The process -- called the convective process by meteorologists -- continues: more and more water vapor turns into liquid, and the moist air warms up even more and rises higher and higher. A thunderstorm results.
WHAT CAUSES LIGHTNING? As more and more water droplets collide inside a cloud, their atoms bounce off each other more forcefully. This knocks off electrons. The ousted electrons gather at the lower portion of the cloud, giving it a negative charge, while the upper part of the cloud becomes positively charged. Eventually the growing negative charge becomes so intense that electrons on the Earth's surface are repelled and burrow deeper into the Earth. The Earth's surface becomes positively charged, and hence very attractive to the negative charge accumulating in the bottom of the cloud. All that is needed is a conductive path between cloud and Earth, in the form of ionized air.
TURBULENT FLOWS: A flow is the continuous movement of a fluid, like water or air, from one place to another. If the air molecules move smoothly in the same direction and at the same speed, this flow is said to be "laminar." Turbulence occurs when the molecules move in many different directions and at many different speeds, so turbulent flows are very common in Nature. How easily a fluid becomes turbulent depends on its viscosity: how much it resists movements. Air currents have low viscosity, so turbulence is quite common in the atmosphere. If you heat air at the bottom and cool it at the top, this convective process will cause it to become turbulent, much like water boiling in a pot. Changes in air pressure can also give rise to turbulent conditions. And when different air masses flow over each other at different speeds they can give rise to beautiful cloud formations.
The American Meteorological Society contributed to the information contained in the TV portion of this report.