5 things you need to know about downbursts

A thunderstorm with winds clocking over 100 km/hr rolled through Kitchener, Ont. on July 27, knocking down trees, lifting up sheds, breaking windows and even ripping two air-conditioning units off the roof of a local business.

The storm also knocked out power for around 6,800 customers. Environment Canada says the damage came from a downburst, or a powerful downward movement of air that can occur during a storm.

To find out more about the phenomenon, we reached Frank Seglenieks, the University of Waterloos weather centre co-ordinator, for some answers. The following Q-and-A has been condensed from an interview on The Morning EditiononMonday with Craig Norris.

1. What is a downburst?

Seglenieks: "A thunderstorm is a big mass of unstable air. So within that unstable air, you have hot air that wants to rise and cold air that wants to drop. So when we think of a thunderstorm, we think of the big tall clouds and that is the warm air going up, but again, what goes up must come down. When that air cools, it wants to go as low as possible, and when it comes down we get downdraft.

Now typically when we feel a thunderstorm, we associate that with some wind, and these aredowndrafts coming down from the thunderstorm. Every once in a while we get a downburst, and thats just a particularly strong downdraft comingfrom this thunderstorm."

2. What makes a downburst so powerful?

"[It] might be coming because the warm air got particularly high and it got really cold and managed to come down. Its also possible that maybe over Kitchener on Saturday, there was a little area of, lets say, drier air, and when that cold air hits that drier air its got a bit of moisture in it, that moisture evaporates, makes it even colder and that air comes down evenfaster.

When it comes down faster its of course going to hit the surface of the earth, and once it hits the surface of the earth its got to spread out. And thats where you can get these winds that, as opposed to a typical thunderstorm [with] maybe winds of 40-50 km/hr, now youre getting these winds of over 100 km/hr."

3. Why do downbursts cause so much damage in such a small area?

"You just get these very small pockets where this air just gets supercooled and that supercooled air comes down really quickly over a very small geographic area, as happened on Saturday. Some streets got their trees totally ripped apart and even a couple of blocks away there was no damage. So its just the way the wind works, and youve just got these small little pockets of really intense, really cold air that really want to go fast.

4. Are downbursts happening more frequently now than in the past?

"People dont like to hear it but generally in the meteorological terms we like to have about 30 years of data before we start to really make some conclusions about the trends. But people have been looking at stable weather stations that havent changed over the past 50 or 70 years and if you over the last few decades there is some evidence that, not only in Southern Ontario but in all of North America, the incidences of more extreme weather seem to be rising over the past decade compared to the decade before. But its going to take us a little while longer to get more data like that before we can really start to make some conclusions about overall trends.

5. Whats the difference between a tornado and a downburst?

In a tornado, the wind is whirling in a vertical vortex, sucking air and debris into the funnel. In a downburst, the air is pushed outwards and away. Downbursts themselves can be further divided into microbursts and macrobursts, depending on the size of the area damaged by wind and the length of time the wind blows. According to the National Oceanic and Atmospheric Administration (NOAA) downbursts are much more frequent than tornadoes, with about 10 downburst damage reports for every single reported tornado. A sure sign of a downburst is a straight line of damage, or trees downed in the same direction the wind was blowing.