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. WH