Thunderstorms and Seasons
One of my students observed that he sees more
thunderstorms here in Pittsburgh, Pa, occurring more often in
spring/summer than in winter. He wanted to know why. I am thinking that
warmer air holds more moisture, therefore more storms result in summer
and in winter we get snow from accumulated moisture. He also wanted to
know why he does not see much lightning during winter storms. Please
explain. I teach environmental science and we happened to be doing a
unit on weather and climate. Thanks! Pam K.
Lightning is caused by a buildup of charge between clouds and the
ground or between different clouds. This requires a rapid air flow which
is more likely in the summer than in the winter, at least in temperate
zones like PA. Having said that, I have witnessed lightning during a snow
storm. This rather rare event occurs when a huge cold front moves rapidly
out of Manitoba across the Plains that has a late warm, wet low pressure
area that has moved in from the Gulf of Mexico. This causes an inversion
that can give rise to a rather spectacular, but short lived, display of
lightning and thunder. For a good description of the "lightning physics"
see Richard Feynman's "Lectures on Physics" Vol. II, chapter 9.
Thunderstorms do occur more often in Spring/Summer because
the conditions are more favorable for their formation
then. A thunderstorm forms when air can be "convected"
easily; in other words, when the rate of temperature decrease
with height (called the temperature gradient) is large.
This occurs during warmer weather because the ground is warm.
Energy and water vapor are released from
the warm surface and rise in buoyant plumes, which, when
combined and organized, form a thunderstorm cell.
In the winter the temperature gradient with height (and with
horizontal distance as well) is smaller and weather systems
are proportionately weaker. Sometimes a thunderstorm will
occur in winter, embedded in a cold front where the air tends
to be rising more strongly. However, the air cannot rise
as rapidly because of the small vertical temperature gradient
and rarely produces a strong thunderstorm.
Lightning occurs only in thunderstorms, which are rare during
winter in the northern part of our country. Lightning can occur
only because rapidly rising and falling water droplets and ice
crystals in thunderstorms cause electric charge to be created
and accumulated in different parts of the thunderstorm. More
charge is created in stronger thunderstorms, few of which
can be formed in winter; thus little lightning in seen in the
David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012