August 2, 1985. A hot, muggy afternoon in Dallas-Fort Worth. Delta Flight 191, inbound to DFW International airport from Fort Lauderdale, has to change course multiple times to avoid thunderstorms.
At 6:05 PM, the Lockheed L-1011 airliner essentially falls out of the sky on final approach, crashing a mile short of the runway and killing 137 people including 8 crew and the driver of a car on the ground.
Ultimately, the National Traffic Safety Board (NTSB) found the cause of the crash to be microburst-induced wind shear. The microburst reduced airflow across the wings, and the reduced lift caused the plane to suddenly lose altitude.
Anatomy of a Thunderstorm
Thunderstorms are one of nature’s most amazing creations. They start to form when warm, moist air rises, bubbling up through colder air above. We’ve all seen those puffy white cumulus clouds that form on muggy spring and summer afternoons.
As a storm grows, it begins to breathe. An inflow pulls warm air upward into the storm. Cold air falls to the ground as a downdraft. High in the storm, moisture gathers into rain. If the inflow updraft is strong and upper air cold enough, the rain freezes, forming hail.
Rain follows the downdraft, forming a distinctive rain shaft. Static electricity is generated by wind and rain within the storm. When that charge grows large enough, lightning slashes between earth and sky, followed by thunder.
A small percentage of thunderstorms will develop a rotation, and that rotation can cause the inflow to form a funnel. When that funnel extends from the cloud to the ground, we call it a tornado.
Eventually, thunderstorms die. The cold air above falls to the ground as the billowing storm cloud collapses and dissipates. The air temperature at ground level can drop by ten degrees or more due to these cold downdrafts.
Sometimes, the downdraft from a collapsing storm will be fairly small, causing the winds to be focused, like the nozzle on a garden hose. When these winds hit the ground, they “splash” out in all directions, away from the storm.
These small (2.5 miles or less), focused downdrafts are called microbursts. Winds from a strong microburst can exceed 100 mph — as strong as an EF1 tornado. In fact, storm damage from microbursts are often thought to have been caused by tornadoes.
Predicting Microbursts
Several aircraft crashes have been attributed to downbursts, and meteorologists have struggled to find ways to predict these elusive phenomena. Doppler radar can show when a microburst is happening, but nothing can yet predict them in advance.
Any time strong thunderstorms form, microbursts are a possibility. Because tornadoes and microbursts are so hard to predict, you may have a very short time to seek shelter.
Your best protection will always be preparation. Decide on your safe space (or get a storm shelter). Make a plan and practice it until everyone knows what to do. And pay attention when severe storms are heading your way.
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