Wednesday's line of severe storms in central Virginia produced an unusual cloud formation. It's called a mammatus cloud, and it often forms on the underside of cumulonimbus cloud.
The clouds appear very threatening, but are often a sign of a weakening storm.
Mammatus are pouch-like clouds that protrude down from the bottom of a thunderstorm's anvil cloud. The storm's anvil consists of ice crystals but, as it spreads out at the top of a thunderstorm, some of the ice begins falling and cools the clear air below.
Warm air rises from the surface and collides with the sinking cool air. The result is small pocket-like clouds that form on the underside of the storm cloud.
While the conditions at the time of the mammatus cloud formation may be quite violent (hail, rain and wind), in most cases, mammatus can be found beneath the anvils of dying thunderstorms.