Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
LOUISVILLE, KY (WAVE) - Summer brings warmth, sunshine and of course those typical afternoon storms. Those afternoon thunderstorms come with a little-known threat of their own: microbursts.
Microbursts are localized downdrafts, or columns of sinking air, inside thunderstorms. They are usually around two and a half miles wide and are common in the southeastern United States during the summer months.
These areas of rapidly descending air can be dangerous to aircraft and those on the ground.
Inside a thunderstorm, water droplets and hail are suspended in a rising column of air called an updraft. If an updraft is strong enough then precipitation can be suspended at the top parts of a cloud. When evaporative cooling occurs inside a storm it weakens the updraft. Evaporative cooling happens when raindrops evaporate as they fall through a layer of dry air thus cooling the air as they absorb the energy necessary to change from liquid droplets to water vapor. Once that happens, the storms can't keep the rain or hail up so this core plummets to the ground an spreads out in every direction causing straight-line wind damage.
The area first hit by the microburst sees the highest winds and the greatest damage.
There are two types of microbursts: wet and dry. Wet microbursts come with significant amounts of precipitation. Dry microbursts occur when the downdrafts sinks away from the cloud and precipitation evaporates inside the downdraft and speeds it up.
Winds caused by microbursts can reach up to 100 mph or more; this is basically equal to an EF 1 tornado. These winds can level trees and cause significant damage to homes and businesses.
When there's a threat of a microburst, the National Weather Service will issue a Severe Thunderstorm Warning.
Meteorologists can forecast for the threat of microbursts six to 12 hours before the storms develop. As we forecast for microbursts, meteorologists look for a few things: instability, high precipitable water (plenty of moisture in the atmosphere/high humidity), dry air in the middle levels of the atmosphere and strong winds within that dry layer.
When there are storms in the area and a threat of microbursts, meteorologists will look for converging air, or air coming together, in the middle levels of a storm; this is called a mid-altitude radial convergence (MARC) signature.
Microbursts only live for a short time. They can even occur between radar scans! This makes the lead time for Severe Thunderstorm Warnings in some cases very short, however, plenty of research is being done to improve that. Learn about how the National Weather Service surveys damage from a microburst versus a tornado by clicking or tapping here.
When a microburst reaches the ground, the air that spreads out can be picked up on radar.