What were those flying saucer-shaped clouds?

Thunderstorms rolled through Madison between 8:30 and 9 a.m. Thursday May 16th. Stunning “flying-saucer” clouds accompanied the dark skies of the leading edge of the storm.

These dark, protruding clouds are “shelf clouds.” The clouds look like a shelf and in this case appeared as a stack of shelves. They are one of several distinct visible features of a vigorous thunderstorm complex.

Stunning “shelf clouds” along the northern shore of Madison’s Lake Mendota on May 16.
Credit: Anne Pryor

Air circulates throughout a thunderstorm, some as very turbulent pockets of air. There are also streams of upward and downward moving air. The upward moving air in a thunderstorm is known as the updraft, while downward moving air is called the downdraft.

Air that is cooler than its environment tends to sink as long as it can stay cooler than its surroundings. Sometimes prior to a thunderstorm rain you may feel a blast of cool air. This is the downdraft spreading out as it hits the surface.

The dense, cold air of the downdraft forms the gust front at the surface and flows out ahead of the storm. The gust front can lift the warm, moist air near the ground, forcing it upward to form updrafts in developing thunderstorms.

Clouds are sometimes observed above the gust front. The shelf cloud is one such cloud. It forms as the gust front forces air near the surface to rise.

A shelf cloud looks very ominous but does not produce damaging weather by itself, although it can precede severe weather by a few minutes. This was the situation on Thursday, and the storms produced brief downpours and wind gusts to around 30 mph.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic Sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.
Category: Meteorology, Phenomena, Uncategorized

Comments Off on What were those flying saucer-shaped clouds?

Comments are closed.