Shelf Cloud and Weather Data.

On the 24th June 2005 I witnessed the cloud formation of a shelf cloud.

A shelf cloud is a low-level horizontal arcus-type cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front. The leading edge of the shelf is often smooth and at times layered or terraced. It is most often seen along the leading edge of an approaching line of thunderstorms, accompanied by gusty straight line winds as it passes overhead and followed by precipitation. It is an extension of the main cloud and the underside is concaved upward, turbulent, boiling, or wind-torn. Shelf clouds are formed when the rain cooled air beneath a thunderstorm sinks, hits the ground, and fans out in all directions. Some of this rain cooled air surges east, lifting and condensing the moist air ahead of the storm, and producing a shelf cloud.

While shelf clouds look very menacing, they are not wall clouds or funnel clouds and rarely produce tornadoes.

Above: The shelf cloud approaching. (07:15 ISO 400 F16 1/45s 19mm)

Above: The Shelf cloud is now going away from us leaving the Mammatus Cloud behind to follow.
(08:05 ISO 800 F16 1/180s 19mm)

Above: Another view of the Shelf cloud now going away from us leaving the Mammatus Cloud behind to follow.
(08:05 ISO 800 F16 1/180s 19mm)

Below is the Davis Weather Station data, before, during and after the event. The light gray area is showing the main changes in the weather data as the shelf cloud approached and passed by.

Below is the lightning data for the time the shelf cloud passed.