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What Is a Squall Storm?

The National Oceanic and Atmospheric Administration's National Weather Service defines a squall line as "A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms." Of the three major classifications of thunderstorms, only the squall line storm is a multicell storm. The other two types are single-cell and supercell thunderstorms.

Appearance of a Squall Line Storm
- Squall line storms are linear in nature and can extend over 500 miles in length. Moving ahead of the squall line is a triangular or wedge-shaped cloud known as a shelf cloud. At the rear of the shelf cloud is a denser formation called a gust front, which is essentially the boundary that separates the gusty winds, which proceed a squall storm, from the thunderstorm itself.
What Causes a Squall Line Storm?
- When the lowest levels of a large volume of air, called an air mass, rise, they are usually cooler than the air around it and will soon sink again. However, an unstable air mass forms when this air is warmer or more humid than the air around it. This unstable air, in conjunction with an approaching cold front, can result in the formation of a squall line.
The Behavior of a Squall Storm
- Each cell of the squall line will usually pass rather quickly. But because the squall line is made up of many cells, a squall line storm often lasts for many hours. A squall storm usually has high winds and heavy rainfall. Although the squall storm may also produce up to golf ball-sized hail, this type of storm rarely produces anything more than a very weak tornado.
Where Do Squall Storms Occur?
- Being the most common type of thunderstorm, squall line storms can occur almost anywhere in the world. More squall line storms occur in the Midwestern United States -- particularly in Missouri and Kansas -- than any other type of thunderstorm.

Nature