We’ve all heard weather reporters warn “a line of thunderstorms is moving through our area.” It’s a summertime routine in some areas. What the television reporter is describing is technically known as a “squall line.” Squall lines can occur at any time of year, but they are most common in spring and summer anywhere east of the Rocky Mountains. They can form over the ocean as well and a re common in parts of the world that have a “monsoon” season of heavy rains.
A typical squall line forms at the leading edge of a moving cold front pushing into a region of warm, humid air. The colder air of the advancing front is denser than the warm air ahead of it, and creates atmospheric instabilities as the two air masses collide. Updrafts form that lift the warm, moisture-laden air high above the earth. This is exactly what is required to trigger the formation of thunderstorms. At high altitudes, the moisture condenses and then freezes, forming the typical anvil-shaped cumulus clouds of thunderstorms.
What defines a squall line is that the individual thunderstorms have this collision of air masses at different densities and temperatures as a common lifting mechanism for propelling warm, moist air into the higher elevations.
The thunderstorms associated with a squall line are short-lived, like all thunderstorms. However, as the cold front moves, it continues to push into more warm, humid air and can continue to generate thunderstorms for hours, or even days, across stretches of hundreds of miles. In many cases, the storms are moderate, bringing only blustery winds, rain, and lightning.
That is not always the case. Some of the thunderstorms can be very intense, and sometimes deadly “supercell” thunderstorms develop in a squall line and can produce tornados. This was the case just a couple of weeks ago in Atlanta, Georgia. During the evening of March 19, a cold front and its squall line of thunderstorms began moving over the city. A supercell produced an F2 tornado with estimated wind speeds of around 150 miles per hour that cut a swath of destruction directly through the downtown area. Windows were blown out of skyscrapers, hundreds of buildings were severely damaged, and dozens of people were injured in a matter of minutes. Fortunately, no one was killed, despite the fact that the tornado damaged the city’s Phillips Arena where an NBA basketball game was in progress.
Squall lines pose less of a threat today than they used to, thanks to modern Doppler radar systems. These sophisticated radars can “see” the shifting masses of air within a supercell and provide early warning of possible tornadoes and high winds, saving many lives.
