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Name: Marc
Status: student
Grade: 9-12
Location: OR
Country: N/A
Date: 5/31/2005


Question:
A few days ago, there was a cold unstable air mass that moved through the Pacific Northwest. I was watching a nearby growing cumulus cloud visually and on NWS radar. The storm produced a single rumble of thunder before dissipating. So I was wondering, under "perfect" conditions, how big (area, height, etc.) must a cumulus cloud be in order to produce at least a single bolt of lightning?


Replies:
There is no easy answer to your question. Lightning has been observed when no clouds are present -- (admittedly this is a rare event) and even during snow storms. Sometimes lightning strikes cloud to earth, but the other direction is also occurs. Cloud to cloud lightning is possible, and even cloud to ionosphere discharges are believed to occur. As common as lightning is worldwide, it is a difficult phenomenon to study. It is only fairly recently with the advent of satellite monitoring that a global distribution of lightning events has been possible.

For some interesting reading I would suggest the book "The Lightning Discharge" by Martin A. Uman, published by Dover Publications (2001) [ISBN: 0-486-41463-9]

A Google search on the term "lightning bolt out of the blue" also will lead you to many interesting sites on the subject including a recent article in Scientific American.

Vince Calder


Dear Marc-

Lightning is a complex phenomena, and not totally understood by meteorologists in every respect. It is caused by charge separation in clouds. Factors affecting this charge are temperature updraft velocities, moisture content of the air, and others. But, a rule of thumb used by forecasters is that cloud tops must reach 20,000 feet for lightning initiation in a thunderstorm cloud. In wintertime, these heights can be lower, because the temperatures aloft are cooler, which affects moisture particle size and other factors. Convective cloud tops in the winter may be as low as 12,000 feet and produce lightning. These are pretty rare however.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO.


Marc,

Generally, taller thunderstorms produce lightning because a build up of charge in the cloud must occur. The charge build up is created by the motion of hydrometers (rain, hail, etc.), which is enhanced in taller thunderstorms. However, thunderstorms with tops no greater than 35,000 feet can produce lightning because of vigorous growth. This kind of thunderstorm is commonly embedded in winter cold fronts.

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


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