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Downbursts...

Wind shear...Downburst...Microburst...
Damaging Winds... 


What is wind shear?

 

 Outflow from a thunderstorm  - the change in direction and speed is referred to as wind shear
FAA Image

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.

 

I've heard the term downburst and microburst...what's the difference?
  • 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 macro burst shows that both can cause extreme winds.

Microburst

Damaging winds extending 2 1/2 miles or less

Lasts 5 to 15 minutes

Can cause damaging winds as high as 168 MPH!

Macroburst

Damaging winds extending more than 2 1/2 miles

lasting 5 to 30 minutes

Damaging winds, causing widespread, tornado-like damage, could be as high as 134 MPH!

 

How do downbursts happen?

 

Simplified model of a downburst showing descending cold air
FAA Image

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!

 

How are downbursts different from tornadoes?

 

A comparison of the inflow around a tornado and outflow associated with a downburst
Graphic by T. Fujita
Downburst


The key difference is in two words - IN and OUT!

IN - all wind flows INTO a tornado. Debris is often laying at angles due to the curving of the inflow winds
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

 

In this photograph, trees are blown down in a straight line - a very strong indication of a microburst as opposed to a tornado.

Photo of trees blown down in parallel lines
Graphic by T. Fujita
Downburst

 

How frequently do downbursts occur?

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

 

Tornadoes

Average of 800 per year in the U.S.

Thunderstorms

Average of 100,000 per year in the U.S.

What visual clues should I look for with downbursts?

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.


Time series photo showing dust picked up by downburst - frames show distinct roll feature
Credit: NOAA Photo Library, NOAA Central Library

 

Why are downbursts so dangerous to airplanes?

The rapid change in wind speed and/or direction poses a very real threat to airplanes during take-off and landing.


Approach to landing through a downburst
NOAA Image

During landings-
1.The airplane begins the descent
2. flying into a strong headwind
3. a downdraft
4. and finally a strong tailwind
5. represents the extreme situation just prior to impact


Take off through downburst
NOAA Image

During take-offs -
1. The pilot experiences a headwind and increased aircraft performance
2. followed by a short period of decreased headwind
3. a downdraft
4. and finally a strong tailwind

 

Wind speed trace from Andrews AFB showing peak gust of 130+ MPH associated with a downburst

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 MPH in this microburst.

The peak gust was recorded at 211 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 MPH. Downburst winds can exceed 165 MPH
  • A loud "roaring" sound
    • Wind speeds of 75+ MPH will often sound very load - leading some to believe they heard a tornado when if 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 damage was caused by a tornado or a downburst is to fly over the area and look down on the damage path.

 

A SEVERE THUNDERSTORM WATCH are issued when conditions are favorable for severe storms (wind gusts of 58 MPH or more or 3/4 inch diameter hail or larger).

A SEVERE THUNDERSTORM WARNING are issued when wind gusts of 58 MPH or greater are imminent (or large hail). In addition, AVIATION ADVISORIES are issued for Low Level Wind Shear for pilots.

Local radio and television stations and NOAA Weather Radio broadcast severe weather warnings
NOAA Image

Severe Thunderstorm warnings are sent to local radio and television stations and are broadcast over your local NOAA Weather Radio serving the warning area. These warnings are also relayed to local emergency management and public safety officials who can activate local warning systems to alert communities to the danger.

Contact us if you'd like a free copy of Thunderstorms, Tornadoes, Lightning ... Nature's Most Violent Storms - a preparedness brochure produced by the National Weather Service, Federal Emergency Management Agency, and the American Red Cross.