Downburst

A graphical representation of a downburstWind Shear

Wind shear is the meteorologist’s way of describing a rapid change in either wind speed or wind direction over a short period of time or distance. Wind shear can describe the changes either horizontally (along the Earth’s surface) or vertically.

Downbursts / Microbursts / Macrobursts

A "downburst" is a strong downdraft which causes damaging winds on or near the ground.

The term "microburst" describes the size of the downburst. A comparison of a microburst and the larger macroburst shows that both can cause extreme wind.

Microburst

  • Damaging winds extending 2.5 miles or less
  • Lasts 5 to 15 minutes
  • Can cause damaging winds as high as 168 miles per hour

Macroburst

  • Damaging winds extending more than 2.5 miles
  • Lasting 5 to 30 minutes
  • Damaging winds causing widespread, tornado-like damage that could be as high as 134 miles per hour

A graphical representation of wind speeds during a downburstDownburst Life Cycle

Cold air begins to descend from the middle and upper levels of a thunderstorm (falling at speeds of less than 20 miles an hour)

As the colder air strikes the Earth’s surface, it begins to "roll" much like water as a boat moves through it. As the colder air "rolls" out, it is compressed causing winds to increase dramatically - at times even stronger than tornado winds!

A graphical representation of the difference between a microburst and a tornadoDownbursts vs. Tornadoes

The key difference is in two words: in vs. out

  • In: All wind flows into a tornado. Debris is often laying at angles due to the curving of the inflow wind.
  • Out: All wind flows out from a downburst. Debris is often laying in straight lines (hence the term "straight-line winds") parallel to the outward wind flow.

Frequency

Downbursts are much more frequent than tornadoes - in fact, for every 1 tornado, there are approximately 10 downburst damage reports!

Type of Wind EventAverage Occurrences per Year in the U.S.
Thunderstorms100,000
Tornadoes800


A series of photographs showing a microburst picking up dust and dirtVisual Clues

This series of photographs shows a microburst picking up dust and dirt - making the "roll" very easy to identify

Unfortunately, you can’t look at a thunderstorm and "see" if it’s going to be severe. Doppler radar is able to "look" inside the thunderstorms and "see" the movement of air - giving the meteorologist indications of microbursts and allowing them to issue warnings.



Downbursts & Airplanes

During Take-Offs

  1. The pilot experiences a headwind and increased aircraft performance
  2. A short period of decreased headwind
  3. A downdraft meets the plane
A graphical representation of a microburst affecting a plane takeoff
A graphica representation of how downbursts affect airplanes

During Landings

  1. The airplane begins the descent flying into a strong headwind
  2. A downdraft meets the plane
  3. A strong tailwind meets the plane
  4. An extreme situation just prior to impact/landing

Microburst Wind Speed GraphDowndbursts in History

In August 1983, the strongest microburst recorded at an airport was observed at Andrews Air Force Base in Washington DC. The winds speeds may have exceeded 150 miles per hour in this microburst.

The peak gust was recorded at 2:11 pm, 7 minutes after Air Force One, with the President on board, landed on the same runway as the microburst was recorded!

Why Downbursts are Often Mistaken for Tornadoes

  • Both can have very damaging winds

    • Tornado winds range from 40 to over 300 miles per hour.

    • Downburst winds can exceed 165 miles per hour

  • A loud, roaring sound

    • Wind speeds of 75+ miles per hour will often sound very loud - leading some to believe they heard a tornado when in fact they only heard a straight-line wind.

  • Trees were "twisted" off - so it must have been a tornado

    • This is one of the most common mistakes. The fact that trees were "twisted" off doesn’t necessarily mean a tornado has gone through. If you could draw a line straight down a tree, you’d see that the tree isn’t exactly alike from one side to the other. Differences in limbs and leaves may cause the tree to have more wind resistance on one side than the other. The tree begins to "twist" (much like a stop sign "twists" in strong winds). If wind speeds are high enough the tree will begin to tear apart in a twisting motion - even though the winds are relatively straight!

The best way to determine if the damage was caused by a tornado or a downburst is to fly over the area and look down on the damage path.

Watches & Warnings

  • Severe Thunderstorm Watches are issued when conditions are favorable for severe storms (wind gusts of 58 miles per hour or more or 3/4 inch diameter hail or larger).
  • Severe Thunderstorm Warnings are issued when wind gusts of 58 MPH or greater are imminent (or large hail).
  • Aviation Advisories are issued for low-level wind shear for pilots.