Ask A Scientist© Weather Archive

name         Dave
status       other
age          50s

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/

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
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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
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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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