Shelf Clouds: One of Nature's Most Alarming and Awesome Phenomena.
Shelf clouds are a stunning feature of many spring and summertime thunderstorms that often pack more bark than bite. Other than for their incredible beauty, shelf clouds are usually on the news, because they tend to freak people out.
A shelf cloud is a low-hanging, well-defined, wedge-shaped formation that occurs along the leading edge of a gust front in a thunderstorm. Shelf clouds most often form just ahead of intense lines of thunderstorms.
Even though they look ominous and people often mistake them for tornadoes, shelf clouds themselves are harmless. What they indicate, however, is potentially more dangerous.
intense lines of thunderstorms. These storms, called squall lines or bow echoes, tend to produce damaging winds when they hit. The most well-defined and photogenic shelf clouds occur with the most intense type of severe thunderstorm.
The main circulation of air in a thunderstorm occurs within an "updraft" and a "downdraft." Updrafts feed warm, moist air into the thunderstorm to provide it with the energy it needs to survive. A downdraft consists of the dense, rain-cooled air that sinks to the surface underneath a thunderstorm.
The air from a downdraft pools up at the surface beneath the storm in what's known as a "cold pool." This cold pool can do one of two things — it can either choke off the updraft and kill the storm, or it can start moving out ahead of the storm, tilting the updraft and letting the storm begin to race along the landscape.
As the cold pool begins to race away from the thunderstorm (now called an "outflow boundary" or a "gust front"), the storm's updraft tilts along the outflow's leading edge, allowing the storm to continue to ingest warm, moist air as it moves along in the direction of its outflow.