Dear Tom,
I have recently moved from Seattle to Chicago. The differences between the two cities are considerable, but what especially fascinates me is the number of thunderstorms that occur here. It hardly ever thunders in Seattle but Chicago gets pounded. Why?
Stan Whillow, Chicago

Dear Stan,
On average, Seattle experiences seven days per year with thunderstorms compared to 38 days here in Chicago. It is a distinction Seattle shares with all communities up and down the West Coast of the United States and Canada. And not only are thunderstorms more than five times as likely to occur here than in Seattle, they are also much more vigorous. Thunderstorms that sweep through Chicago are sometimes accompanied by heavy rain, hail, frequent lightning strikes, strong and gusty winds and, on rare occasions, tornadoes.

Thunderstorms are produced by cumulonimbus clouds, the vertically built clouds that typically tower to 45 thousand feet and occasionally up to 65 thousand feet above their bases, which are usually below three thousand feet. Cumulonimbus clouds contain churning updrafts and downdrafts, and they become electrified when water droplets and ice particles intermingle in complex ways. The mix of water and ice is crucial in setting up lightning in and around the cloud.


Cumulonimbus clouds form best when cool air overlays warm air. This occurs most frequently over warm southern land masses and, in the warmer portion of the year, over inland areas of North America. Seattle, however, sits relatively far north. In addition the Pacific Ocean provides coastal North America with a flow of cool air that discourages the formation cumulonimbus clouds. This ocean cooling plays a huge role in reducing thunderstorm numbers on the West Coast of the United States.