On thunderstorms and straight-line winds

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With the return of summer-like heat earlier this month, we also saw the return of severe summer weather. Plenty of thunderstorms rumbled across the Prairies, bringing examples of all of the different types of severe weather we can expect to see. There was plenty of lightning, hail, some really strong winds, heavy rains and, last but not least, a few possible tornadoes!

So far this year we’ve discussed lightning and hail, which means we only have three severe weather phenomena left: heavy rain, strong winds and tornadoes. Probably the most life-threatening form of severe summer weather is strong winds. Most people associate strong thunderstorm winds with tornadoes. In reality, tornadoes only account for a small percentage of wind damage caused by thunderstorms, but when tornadoes do occur, the damage is usually truly incredible!

Radio weather lacking

Before we start to discuss thunderstorm winds, I have a little bit of an issue I need to get off my back. I decided to take an extra-long weekend and head out camping for a few days. Knowing I would be “off the grid” and not able to check out radar images and other near-real-time weather information, I checked the weather in detail before I left, then relied on good old radio-based weather forecasts.

Well, I’m not sure who I should be more upset with: Environment Canada coming up with radically different forecasts every six hours, the poor weather information given out by radio stations, or myself for not being able to read the weather better. Needless to say, when it was supposed to be sunny or partly cloudy it was raining with thunderstorms, and when it was supposed to be raining with thunderstorms it was sunny to partly cloudy. All in all, it was a tough camping trip weather-wise.

After thinking about it over the past couple of days I’ve become more and more upset with the quality of the weather forecasts put out by radio stations. Now I know I should have had a weather radio with me, but I didn’t, and to tell the truth, I’m not sure if I would have been close enough to the nearest station to pick up anything. What has got me upset about the radio weather reports is the fact that most radio personalities have no idea about the weather and if they simply read Environment Canada’s forecast, that would be OK (they’re not the best as it is, but I won’t go there), but instead they tend to “clip” the forecast and make it shorter, leaving out what they don’t think is important information in order to keep it short and concise but often missing key pieces of information.

OK, I feel a little better now, back to the topic of thunderstorms and wind.

Push and pull

To start our discussion about wind and thunderstorms we need to realize there are two types of destructive winds: straight-line winds and tornadoes. We will discuss tornadoes in the next issue. So let’s take a look at what causes straight-line winds and in which part of the thunderstorm we tend to find them.

To understand where these straight-line winds come from you first have to remember how a thunderstorm forms. A storm forms when warm air rises, lifting tonnes of moisture into the air; this moisture then condenses and forms raindrops that eventually fall back to the ground. Now, if you have ever been near a hose spraying water, you know the spray of water pushes the air around it along with it, and the same thing happens within the thunderstorm. The falling rain pushes and pulls the air along with it, and when it hits the ground it has to spread out. The spreading out of these downdrafts of air in a storm can create some very strong winds, especially when different downdrafts collide and merge together. Winds from these downdrafts can be as strong as 90 kilometres per hour, but are typically in the 50- to 70-km/h range. We often see this type of straight-line wind out in front of a thunderstorm and it can be been seen by the accompanying cloud wedge or roll cloud produced by these strong winds.

These downdrafts can also tap into very strong winds high up in thunderstorms. Large, strong thunderstorms often have jet streaks associated with them. These are similar to jet streams in that they are rivers of fast-moving air and these jet streaks are often helping to fuel the storms. Within the thunderstorm, when a strong downdraft occurs, it can hit this jet streak and basically force it to the ground. This very fast-moving air then fans out along the ground just like a regular downdraft, only in this case, producing winds in excess of 100 km/h. It’s these straight-line winds that tend to produce the most damage.

Next issue: Tornadoes!

About the author

AF Contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the University of Winnipeg. He operates a computerized weather station near Birds Hill Park, Manitoba.



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