Whenever a thunderstorm produces damaging winds, many people immediately assume that a tornado is responsible. However, the culprit is usually less exotic, but often just as destructive. Strong thunderstorms will often produce powerful straight line winds, which can be up to 100 miles per hour. As winds flow down from the clouds and spread outward along the ground, this is called a downburst. All thunderstorms have updrafts and downdrafts, but a downburst is particularly strong and destructive downdraft.
Thunderstorms are relatively small and don’t last long. The lifetime of a thunderstorm is on the order of an hour, from birth to death. It can be just a matter of minutes between the first formation of the cumulus cloud to the first lightning strike. In the life cycle of a thunderstorm, the first phase of formation is dominated by updrafts as the storm grows vertically into the mature stage. In the latter stages of the thunderstorm, the decaying phase, air that has been lifted high up into the atmosphere is now cooler than the surrounding air, and it will accelerate toward the ground. Once the higher winds hit the ground, they spread out radially from the center of the downdraft. Damage from a downburst can be distinguished from tornado damage because tornado damage will not spread out from a central point.
This downward moving air, or downburst, is further divided into two categories, depending on their size. A downburst of winds from a decaying thunderstorm is considered a “microburst” if the width of the damaging winds is 2.5 miles or less, with winds that persist for 15 minutes or less. If the downburst is larger than 2.5 miles and damaging winds last for 30 minutes or more, the downburst is considered a “macroburst.”
Microbursts can be particularly dangerous for air travel. If an airplane is approaching an airport for a landing, hitting a microburst can be deadly. Air flowing over an airplane’s wing generates lift, and when an airplane flies into a microburst, more air flows over the wing and generates more lift. When this happens, a pilot may reduce their speed to compensate and maintain the flight level. And as the plane continues to move forward, it moves under the downward moving air directly under the cloud and can be pushed down to the ground. This is one of the multiple reasons flights are delayed when thunderstorms are near the airport.
