PDA

View Full Version : First the thunder, then the rain. . . .



Jason Harris
01-19-2010, 11:05 PM
This may be a very subjective question and only one that relates to a relatively
weak single-cell or multi-cell cluster rather than a fast-moving squall line or supercell, and in particular it's how I grew up with thunderstorms in CA. I recall
how in February, March, or April the clouds would gradually build up as the day warmed, the lower levels would slowly get criss-crossed with relatively unsubstantial stratus, darkening incrementally, greying and sometimes partly transparent but eventually quite threatening (nimbostratus?), and you'd stop and wonder why the heck it wasn't just pouring since it sure was dark enough.

Then, a quick flash, a clap of thunder and the rain began. The storm itself seemed to be virtually stationary, so it wasn't a matter of the rain coming behind the lightning because of the storm's approach; it literally felt like some sort of tension was released by the lightning and thunder, as though it were a necessary step before the rain would be released. Is there any particular process behind this impression of a storm? It doesn't seem to work that way out here in FL or in MI where the storms seemed more often moving and announcing their presence by lightning well before the rain arrived, or there would be some showering without any preamble, perhaps the higher dewpoints vs. the lower ones in CA? Any thoughts? Is there anything at all to the idea that the electricity released from the lightning discharge somehow contributes to a downdraft for rain or something along those lines? Or is this all subjectivity based on the maturing storm once it reaches the height to have lightning is ready to have downdraft?

Larry J. Kosch
01-26-2010, 10:27 AM
The answer, my friend, lies in the positive and negative charges that a thunderstorm holds. Here's a good website that answers questions about lightning:

http://www.usatoday.com/weather/resources/askjack/archives-lighting-science.htm

One good question about lightning:

Q: Where does lightning come from?

A: Lightning comes from differences in electrical charges that occur in thunderstorm clouds or between a thunderstorm cloud and the ground. As a thunderstorm grows to the point where its top extends to above the freezing level, a charge separation begins to develop, with positively charged particles accumulating near the top of the thunderstorm cloud and negatively charged particles near the base.

This charge separation is likely a result of small ice particles and hailstones colliding with each other in the updrafts and downdrafts of a thunderstorm. When the difference in electrical voltage between these charged particles becomes large enough, an electrical discharge occurs in an attempt to equalize the charge separation. This is lightning.

About 80 percent of all lightning is cloud-to-cloud lightning, which occurs within a thunderstorm cloud. The remaining 20 percent is cloud-to-ground lightning, in which a charge separation develops between the negatively charged particles in the cloud base and positively charged particles near the ground.

In your cloud case, you would not have been aware of the lightning that was going on inside the cloud, until that CG (cloud-ground) stroke occurred. The storm cell had generated enough electrical imbalance in the cloud base to set up a charge potential with the ground.

Here's another question:

Q: Last Night our local forecaster described some lightning in our area as "positive lightening". I thought lightening was made up of positives and negatives. Have you ever heard this term and if so, what is positive lightening?

A: Electricity does have positive and negative charges. Lightning scientists have found that most lightning "lowers negative charge to the ground." In other words, during a lightning stroke some of the excess negative charge in a cloud zips to the ground. They have also found that some lightning discharges bring positive charge to the ground. In general, positive lighting has a stronger current and does more damage. Flashes of lightning that hit towers and high buildings such as the Empire State Building are positive. Researchers are trying to discover whether there are any links between the different kinds of lightning and thunderstorms that produce tornadoes. Lightning that comes from the overhanging "anvil" of a thunderstorm is often positive. (2-28-96)

It seems that the CG lightning stroke "drains" the cloud's electrical potential and ability to hold its moisture within its updraft/downdraft circulation. When the drain occurs, the cloud is unable to hold the moisture and the rain begins falling out.

I remembered one occasion years ago when I was doing my farm chores. It was hot, humid and storm clouds were building up. I saw this one small, isolated cloud build up with a nice little tower. I didn't see any lightning until one single CG stroke hit the ground. Then a small rain shower came down from the cloud base. The cloud dissapated shortly after that rain shower.

So there you have it. It's all about the positive and the negative charges! :D