How a Squall Line is Formed

Just the word “squall” sounds ominous, bringing to mind visions of chaos and mayhem. In reality, squalls are sudden violent storms featuring straight-line winds and severe thunderstorms. There can be squalls in the winter but generally, squall lines form during the spring and early summer in the United States.

In weather lore, there are reports of white squalls sinking large ships in mere moments. A white squall is a sudden, violent windstorm at sea. The storm springs up from nowhere, creating white mist and foam in both the air an waves. At sea, visibility becomes non-existent. White squalls are common in the Great Lakes and were responsible for the loss of the Pride of Baltimore off the coast of Puerto Rico in 1986.

Whether at land or sea, a squall is, in fact, a violent storm. On land, squalls tend to form in a line that can be hundreds of miles long. Squall lines are made up of many individual thunderstorms that have organized themselves into a weather freight train. Cloud formations of squall lines resemble a shelf or triangular shaped mass. Squalls are considered a severe weather event and are second only to hurricanes in damages and casualties.

Like many other severe weather events, squalls rely on updrafts and down drafts to generate the energy required for formation. Unstable air masses, well ahead of an approaching front, collide and warmer air rises and condenses to release its moisture in the form of clouds and rain. Squall lines form on the leading edge of a cold front, but are separate from the front. They are composed of a series of thunderstorm cells that can reach speeds up to 60 mph.

A squall is characterized by a sudden increase in wind speed above 18 mph that attains a speed of 25 mph for more than one minute. Squall lines have more intense rain and winds than regular thunderstorms. Embedded inside squall lines are cell thunderstorms than can spark enough turbulence to cause hail, heavy downpours, lightning, and tornado activity. Flash flooding is common during squalls, as are sudden outbursts of wind (down bursts) that occur at or near the ground.

Squall lines don’t only result in thunderstorms but can also form during the winter, creating snow squalls. This line of intense snow activity can produce heavy amounts of snow and gusty winds. Snow squalls are produced by Arctic air moving over a warm body of water, such as the Great Lakes. In the desert, squall lines can form over arid regions. These storms are called haboobs. Down bursts from a haboob cause sand to be lifted into the air, forming a wall of sand that is seen in a sandstorm.