Dry Lightning
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Hi I'm Dave Thurlow and this is The Weather Notebook. As the summer fire season draws to a close, listener Eileen Williams of Grand Haven Michigan has a question about the cause of some of the worst forest fires in history. She listens to the Weather Notebook on Michigan Public Radio from Ann Arbor-Grand Rapids. Here's what she writes.

There are so many fires out west this year and we keep hearing that a lot of them are being started by "dry lightning." Could you please explain to those of us that live in a wet Great Lakes area what exactly "dry lightning" is and what the difference is to the lightning that we experience in a thunder storm with rain?

Well, that's precisely the difference. A thunderstorm with rain produces "wet" lightning I guess you could call it. A thunderstorm without rain produces dry lightning. A thunderstorm without rain actually does have rain; it's just that the rain dries up before it reaches the ground. This type of storm is common in the west where the air is considerably drier than it is generally east of the Mississippi.

Because it's so dry in the west, thunderstorms form much higher in the sky, so high that when the rain falls it often evaporates before reaching the ground. But the lightning generated in the storm, the same way it does in any storm east or west, CAN reach the dry land below, even if the base of storm itself is two, three or more miles high. When lightning does strike, a fire is sparked but there's no rain falling to douse the flames. And this summer the unremitting heat dried the forests to the point were any spark, especially from dry lightning, could turn to a blaze in seconds.

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