Thunderstorms pose a number of familiar hazards, such as lightning and hail. The lesser-known downburst, however, is also a serious threat to life and property.
A downburst is a strong downward current of air that causes damaging winds on or near the ground. They initiate high up in the atmosphere, where relatively dry air is entrained inside of an intense thunderstorm. The dry air evaporates some of the storm’s raindrops, which has a cooling effect. Since this cooler air is denser than the warm air that surrounds it, it sinks rapidly toward the surface. When it hits the ground, it spreads out radially – in straight lines in all directions. Reaching speeds in excess of 100mph, a downburst will knock down trees and other obstacles leaving a trail of debris all facing the same direction.
These straight-line wind events, according to the NWS, can vary in size and duration. When they cover an area less than 2.5 miles, they are referred to as microbursts. These typically last between 5 and 15 minutes. Larger events, known as macrobursts, affect an area greater than 2.5 miles and last from 5 to 30 minutes.
While short-lived, these powerful winds can pose a threat to property on the ground as well as airplanes in the process of taking off or landing.