Downbursts or Straight Line Winds vs. Tornadoes
With the massive bow echo that moved across the region late Tuesday night into Wednesday many people swear it was a tornado. The Doppler radar along with storm surveys of the damage showed there to be nothing but straight line winds. What many people tend to forget is that severe thunderstorms can and do produce winds of 58-125 mph. Which is more than enough to cause widespread damage. Let’s a take a look at some of the radar images from that night. These were taken around 1am from York and Chester counties where the worse damage was noted.
The above are split screens of the reflectivity and velocity data. You see the strong winds in the outflow along the bowing segments which at times were producing winds of 85-90mph. The main areas were producing winds of 70-80 mph. You can also see some broad circulations which are common wherever there are notches in the line. These are not tornadoes because they are too broad and weak but enhanced wind damage is likely in those areas. It also appears that there may have been horizontal rolling going on which is like a weak tornado turned on it’s end and rolling along the ground near the leading edge of the outflow.
(Credit NWSFO Columbia, SC)
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.
Damaging winds extending 2 1/2 miles or less
Lasts 5 to 15 minutes
Can cause damaging winds as high as 168 MPH!
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?
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?
Graphic by T. Fujita
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
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!
Average of 1000-1200 per year in the U.S.
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.
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.
(Credit NWSFO Columbia, SC)