Microbursts an unpredictable weather hazard

COLUMBUS (WCMH) — Ohio experiences an average of 16 tornadoes annually. Yet many reports of wind damage caused by thunderstorms are not tornadic storms.

A lesser-known phenomenon — a microburst — is responsible for pockets of storm damage that can be quite substantial in narrow zones. The difference is that microburst winds fan out in a straight line, compared to the rotary winds of a confirmed tornado (funnel on the ground).

Microbursts are powerful downdrafts (downbursts) that can produce a wide damage path of up to 2.5 miles; macroburst damage extends over greater distances.

Downbursts are caused by the evaporation of droplets as dry air is ingested aloft by a storm, producing powerful downdrafts that spread out beneath a gust front, usually preceding the rain shaft.  Wind gusts can occasionally achieve hurricane force (74 mph or greater). A wet microburst is fairly common in the Eastern United States associated with both entrainment of dry air and a heavy load of water in the storm, which collapses to the ground with a ball of rain and powerful wind gusts.

A special kind of thunderstorm squall line–a derecho–consists of damaging straight-line wind gusts over hundreds of miles.  On June 29. 2012, winds gusted to 82 mph in central Ohio (91 mph at Fort Wayne, Indiana), resulting in more than one million Ohio customers losing power as trees and power lines were toppled by the high winds.

Most microbursts are short-lived, localized events, capable of uprooting trees and tearing off shingles or siding from buildings. A  more serious hazard in past years was the risk of aviation disasters during takeoffs and landings.

An airplane unexpectedly flying into a microburst on the final approach encounters a strong headwind, possibly causing the pilots to pull up on the throttle to regain altitude.  Suddenly, a reversal occurs in the form of a strong tailwind that causes the plane to lose airspeed and stall, with the potential for a crash.

Terminal Doppler Weather Radar (TDWR), a high-resolution scan, detects dangerous wind shear and heavy precipitation, providing crucial information to air traffic controllers in the control tower. Aircraft are immediately alerted to the microburst threat and make the necessary adjustments.

The last U.S. jet crash directly attributable to wind shear happened in 1994, when USAir Flight 1016 went down in the woods near Charlotte Douglas Airport, killing 37 persons and seriously injuring at least 16.  The implementation of on-board and ground-based wind-shear detection clearly saves lives, in concert with lightning location sensors.

The difference between a tornado and a microburst visible on the ground is evident in the nature of the damage; tornadoes leave a twisted path of debris, while microbursts create fan-shaped or straight-line wind damage.





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