By Duane Friend
What is a downburst?
As I wrote this, just a few miles away folks were sawing up downed trees and clearing debris from severe wind damage that occurred in South Jacksonville during a round of thunderstorms on October 24. No tornado was observed either by eye or on radar, so it appears to have been caused by straight line winds. However, the worst of the damage was confined to a narrow area that is over a mile long. The relatively small area of damage may have been due to a downburst, a localized area of strong downward air that hits the ground and spreads out.
What causes a downburst?
Thunderstorms develop as air rises in an updraft. More air is brought in to replace the air that is rising. Many times the air is moist, making the updraft more buoyant. However, on some occasions the air brought in is dry, which causes water droplets in the updraft to evaporate and cool. The cooling may cause the updraft to weaken or stop. Since air that is both cool and dry is heavy, the air sinks rapidly. Sometimes suspended hail and rain come down as well, adding to the wind velocity as these particles drag air down.
Once the air hits the ground, it quickly spreads out. The front edge of the air wraps around itself, adding to the wind speed. A 50 mph wind can become a 100 mph wind in the area where the air develops this rolling. Downbursts may or may not contain precipitation.
Downburst discovery
This phenomena was first described by meteorologist Tetsuya “Ted” Fujita, better known for the Fujita scale of tornadic wind damage. After a commercial jet crashed in the mid-1970s while attempting to land during a thunderstorm, Fujita suspected a strong downdraft caused the crash. His theory was based on previous aerial observations showing a starburst pattern of damage from a central point after thunderstorms had passed through an area. It took some time before his theory was shown to be correct, but now airports are very vigilant for microbursts when thunderstorms are close by.
Predicting a downburst is not easy
Since this phenomenon is small in scale and happens rapidly, it is not currently possible to provide advance warning on where exactly microbursts will occur. Weather radar can detect these as they occur if the radar is not too far away. Certain weather conditions may provide a general likelihood of microburst development, which are in many cases the same conditions for severe thunderstorm development.
For more information on weather phenomena, visit the Illinois Extension “All About Weather” blog at https://extension.illinois.edu/blogs/all-about-weather.
