17th April 2006

Cloud-to-Air Lightning

Source: Excerpt from The Book " Weather "

Distribution: Worldwide, except Antarctica; common in the tropics. Height: Anywhere within the height range of a cumulonimbus cloud. Cause: Electrical discharge between a thundercloud and the adjacent atmosphere. Associated Weather: Heavy rain or hail, strong winds from associated thundercloud. This form of lightning occurs when an electrical discharge takes place between a build-up of one type of charge within a cumulonimbus cloud and an area of opposite charge in the surrounding atmosphere. This type of lightning tends not to be as powerful as cloud-to-ground lightning, and one stroke is normally enough to reduce the difference in charges to below critical levels. As a result, repeated flashes along the same cloud-to-air leader stroke are unusual. Cloud-to-air lightning normally occurs between the air and the positively charged upper regions of clouds. It does also occur in the lower parts of clouds, but usually in combination with the positive flash form of cloud-to-ground lightning, when the cloud-to-air bolts will appear as weaker offshoots from the main flash. This happens because, in the lower layers, the difference in charges between the cloud and the ground is normally greater than the difference in charges between the cloud and the surrounding air. Because cloud-to-air lightning normally occurs near the top of a cumulonimbus cloud, it is often seen from a considerable distance away. If the storm is too far away for the thunder to be heard -- usually over 20 miles (32 km) -- the observer may witness a "Silent Storm". When cloud-to-air or cloud-to-cloud lightning is obscured by clouds, the viewer may see only its flickering reflection in the adjacent clouds. Commonly known as "sheet" lightning, this is actually a simple optical effect rather than another type of lightning.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.