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Name: Dave
Status: other
Age:  50s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
What is the difference between a positive lightening strike and a negative lightening strike? They talk about them at website http://www.intellicast.com/LocalWeather/World/UnitedStates/SouthCentrallight ningLoop/


Replies:
Dear Dave-

Lightning is a fascinating part of weather, and I'll try to explain positive and negative lightning strokes. But first, you should visit this link, and read a detailed, but non-technical explanation of how lightning forms. It is very interesting. Here is the link:

http://www.howstuffworks.com/lightning.htm

The differences between positive and negative strokes is not discussed in detail in the link. Most cloud-to-ground strikes are negative, and a much less common number are positive. The only difference between the two kinds is the reversal of polarities in the cloud base. Normally the negative charge collects in the cloud base, with a corresponding net positive charge in the ground under the cloud. Lightning strikes originating from this configuration are negative strikes.

But if the cloud base becomes positively charged relative to the top of the cloud, the ground below then assumes a net negative charge, and any lightning that develops will be a positive strike.

The lightning detection sensors used by many data observation and collection organizations are able to distinguish between positive and negative strikes, and report them as such. Research is ongoing, to determine if there is a relationship between positive strokes and certain types of severe weather.

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


Dave,

About 90% of all lightning strokes are negative strokes, meaning that they were initiated by a large concentration of negative charge in the cloud-base; this tends to induce an area of positive charge on the ground. The positive lightning stroke is exactly the opposite, with a positive charge concentration in the base of the cloud inducing a negatively charged area on the ground. Positive strokes are most common in severe thunderstorms just prior to tornado formation and are being studied heavily now as possible predictors of severe weather and tornado formation. If we could identify the correlation and timing of positive stroke formation, we may have one more predictive tool to give people early warning of a tornado.

David Cook
meteorologist
Argonne National Laboratory


Volume II of Feynmann's LECTURES ON PHYSICS -- I think Ch. 9 and 10 give an understandable treatment of lightning.

Vince Calder


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