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Name: Pam
Status: educator
Grade: 9-12
Location: PA
Country: N/A
Date: 1/9/2005


Question:
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.


Replies:
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.

Vince Calder


Pam,

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 winter.

David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory


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