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From: mls@husky.uucp (Mark Stevans)
Newsgroups: net.sci
Subject: Re: Question about Electricity
Message-ID: <262@husky.uucp>
Date: Tue, 5-Nov-85 12:24:41 EST
Article-I.D.: husky.262
Posted: Tue Nov  5 12:24:41 1985
Date-Received: Fri, 8-Nov-85 06:16:29 EST
References: <621@hlwpc.UUCP> <662@petrus.UUCP> <714@alberta.UUCP> <554@ttrdc.UUCP>
Organization: Eastman Kodak Company, Rochester, NY
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The question of how atmospheric electricity is generated has been raised by
the referenced article:

> Now the question
> of how the cloud achieves that voltage is a different one and I understand
> there are several explanations (anyone care to elaborate?).

Here is the best theoretical explanation of cloud electrification that I know
of:

Clouds are made up of droplets of water.  If there is a lot of water in the
cloud, larger droplets may fall out of the cloud as rain.  If the cloud is
relatively calm, the droplets usually stay in one piece as they travel about.
In stormier conditions, water droplets break up and recombine as they get
blown about in the cloud.

There are some free electrons in every droplet of water.  They naturally
collect around the outside of the droplet, due to mutual repulsion.  If a
droplet begins to break up into two sub-droplets, the electrons tend to stay
on the larger droplet (statistically and fluid-dynamically speaking, droplets
usually don't break up into exactly equal sub-droplets), because during this
period (envision the process as similar to a yeast cell budding) the larger
sub-droplet has less surface area per unit volume.  We end up with a relatively
positively charged small sub-droplet, and a negatively charged large
sub-droplet.

Due to their greater weight to surface area ratio, the large, negatively
charged droplets tend to sink in the cloud, perhaps hitting the ground as
rain, while the small droplets stay suspended in the air currents.  Since the
free electrons in the cloud are moving to lower altitudes, a potential
difference is generated.

Most lightning strokes are between upper and lower parts of a single cloud,
and not between cloud and ground.  The potential difference per unit distance
increases until it is sufficient to ionize a "finger" of air about an inch
wide.  This highly conductive finger, which typically snakes from a positively
charged region towards a negatively charged region, lengthens at a speed on
the order of a thousand miles per hour.  When the end of the finger gets close
enough to its target, the potential difference between the end of the finger
and the target causes free electrons on the target to arc across the gap into
the ion finger.  This "return stroke" is what we know as lightning.

					Mark Stevans
					ritcv!husky!mls