You hear thunder in the distance, maybe even feel a breeze and smell rain. You look out your window, and you see dark storm clouds in the distance. The news station alerts you of the possibility of storms in your area that afternoon. Severe storms can bring a multitude of sporadic conditions with it, microbursts are one of them.
It all starts with a thunderstorm, but what exactly is a microburst? Microbursts AKA downbursts, are made of wind rushing down to the ground. Once the wind hits the ground, it spreads in all directions. Wind speeds in a microburst can be 60-100 mph, damaging roofs, snapping trees and knocking over power poles. There are two types of microbursts; wet and dry. A wet microburst has its own, unique characteristics. Like the dry microburst, wind rushes out of the storm, hits the ground and spreads in all directions. But unlike the dry microburst, a rush of heavy rainfall accompanies the damaging wind. A dry microburst hits the ground without any precipitation, making them virtually impossible to see unless they kick up dust and dirt at the surface. Dry air entrainment is basically the only process driving these wind events.
What types of areas do microbursts affect? Microbursts tend to affect a small area, no larger than a few square miles in most cases. The intense damage these wind events leave behind can cause residents to think they had a tornado. While weak tornadoes and microbursts can produce similar amounts of damage, there is a marked swirl in tornado debris on the ground when viewed from above, while microbursts produce damage in a starburst pattern, with straight-line winds radiating away from the point of impact.
Microbursts occur through two processes: dry air entrainment and water loading. Dry air entrainment occurs when dry air mixes in with raindrops within a cloud. The dry air causes the drops to evaporate, lowering the air temperature through evaporative cooling. This area of cooler air begins to sink through the thunderstorm and gains speed as it falls. If there is a steep lapse rate (large and steady change in temperatures) beneath the storm, the cool bubble of air will sink faster because the air around it will grow warmer (and less dense) closer to the ground. This rapidly-descending column of air will eventually slam into the ground and spread out in all directions with winds of 60+ MPH, creating the microburst.
(Image credit: NOAA)
Microbursts are a dangerous severe weather phenomenon that can cause great amounts of damage with little or no warning. The best way to protect against microbursts is to pay attention to severe thunderstorm warnings issued by the National Weather Service.
(Images: NWS and NOAA)