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.
Damaging winds extending 2.5 miles or less
Lasts 5 to 15 minutes
Can cause damaging winds as high as 168 miles per hour
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!
Downbursts 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.
Downbursts are much more frequent than tornadoes - in fact, for every 1 tornado, there are approximately 10 downburst damage reports!
Type of Wind Event
Average Occurrences per Year in the U.S.
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
The pilot experiences a headwind and increased aircraft performance
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.