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WEATHER 101: What is a Rain Foot?

A viewer on facebook shared this image of a thunderstorm that occurred over the weekend with a classic looking rain shaft featured in the middle of the photo.  Also of note is the "rain foot" displayed on the right side of the rain shaft. 

Anna Smith New WashingtonThunderstorm producing a rain shaft and associated "rain foot" near New Washington, Indiana.  Image credit:  Anna Smith

The rain foot is the base of the rain shaft that bends away from the main column of precipitation.  

Anna Smith New Washington RAIN FOOT
This bending or deflection of falling precipitation is caused by strong thunderstorm winds that occur as a result of a downdraft that strikes the ground and spreads out in all directions.  This is known as a "microburst".    
242460_10150221038153556_837803555_7064558_5235911_oRain foot associated with severe thunderstorm winds of up to 85 mph in the Oklahoma City area.  Image credit:  NWS Norman, OK
We saw several microbursts in our area on Saturday afternoon that downed trees and snapped power poles.  Particularly hard hit were portions of Oldham County where winds were estimated to have reached up to 75 mph.
Microbursts can create winds as strong as some tornadoes and in some cases can exceed 120 mph!
0603wxfig1lgDiagram of thunderstorm downdraft/microburst.  
By definition, microbursts are small is scale on the order of 2.5 miles in diameter or smaller and are created when pockets of heavy precipitation meet fall through a layer of dry air.
This results in rapid "evaporational cooling" within the thunderstorm downdraft.  Evaporational cooling causes the air to become more dense and makes it accelerate towards the earth increasing in speeed as it goes.  
CharleswylderThunderstorm producing a rain foot illuminated by lightning at night.  Image credit: Charles Wylde
When the rush of wind reaches the ground, it spreads out horizontally causing localized damage to homes, trees and powerlines.  Sometimes microburst winds can become downright destructive! 
Microburstdamageex1Areal view of severe microburst damage to forest over Northwest Arkansas.  Winds were estimated at over 100 mph. 
Microbursts became the focus on intense meteorological study after one caused the crash of Delta Air Lines Flight 191 when 137 passengers died when a Lockheed L-1011 jumbo jet got caught in 80 mph microburst/downdraft winds while attempting a landing at Dallas/Fort Worth International Airport in August of 1985.  
Skip Talbot2Severe storm in Western Kansas producing well defined rain foot.  The whitish tint to the precipitation shaft here is an indicator of hail.  Image credit: Skip Talbot

WDRB Meteorologist Jeremy Kappell

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