Anatomy Of A HEAT WAVE (If One Arrives)

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While a good chunk of North America has been under the grip of an intense heat wave, much of Alberta has been suffering under cool conditions with plenty of rain.

The heat wave started around the middle of the month and brought a few nice days to Alberta and Saskatchewan, but it wasn’t until you reached Manitoba before you really saw the heat. Over Manitoba temperatures soared into the mid-30s. Combine this with dew points in the low to mid-20s and we saw humidex values in the mid- 40s. These hot conditions lasted for nearly a week before the heat wave pushed eastward.

It intensified as it moved east, bringing record temperatures to nearly all of eastern North America. Over the United States, more than 11 all-time record highs were broken during this heat wave, with many of those records being from the notorious 1936 heat wave.

Why am I teasing you with all this talk about heat waves? Well, I am hoping that maybe, if we talk about hot weather and heat, that some of it will move in and bring a nice, hot August to Alberta. So let’s take a little time to take a look at just what makes a heat wave and then cross our fingers and hope that warm weather moves in!

First, how do you characterize a heat wave? The first and most obvious thing is heat or better yet, excessive heat. After poking around a little it seems that excessive heat is defined as occurring when high temperatures are more than 6 C warmer than the long-term normal. For most of Alberta, that would mean high temperatures exceeding 28 C.

One hot day does not a heat wave make, so our second characteristic is duration. Again, after a little digging, it appears that most places set the duration for a heat wave at two or more days.

It’s not the heat, it’s the…

Our next characteristic is humidity. This one is a little tougher to define as you can have a heat wave and have both low and high levels of humidity, but generally, when really epic or intense heat waves are talked about they also include high humidity. This makes sense, as high temperatures plus moisture in the air make the air feel even hotter. This is due to the fact that as atmospheric moisture levels increase, the ability of the air to “take in” more water decreases. One way we cool ourselves is to sweat, and sweating will only cool us if the water on our skin can evaporate. If the air is too humid to evaporate this water then we feel even hotter.

The final characteristic of a heat wave is atmospheric pressure. Nearly every heat wave is associated with regions of high pressure, whether that high pressure is at the surface, the upper atmosphere, or both. This is where we are going to get just a little bit technical.

Most heat waves are associated with regions of upper-level high pressure, or what is often referred to as ridges of high pressure. Upper highs form when air aloft or in the upper atmosphere is converging. This basically causes the air to pile up – think of a bunch of cars all converging from several lanes down to one lane. All that air piling up has to go somewhere.

Fortunately our atmosphere is three dimensional, so some of that air will start to be pushed towards the ground and will sink. As this air sinks it gets compressed, and if anyone has used a compressor or simply a bicycle pump, you may have discovered that when you compress air it gets warmer. This is exactly what is happening in the atmosphere. The descending air is warming.

Descending air is also not very good at letting clouds form. For clouds to form we need rising air not descending air, so upper ridges tend to bring a fair amount of sunshine. Add all this energy from the sun to the ground, combine it with the warm descending air, and you get high temperatures. If the upper ridge stays in place long enough, all the heat will dry out the ground and humidity levels will begin to drop. More of the sun’s energy can then be used to heat the air instead of evaporating water, and temperatures will get even hotter and drier. Then before you know it, we have gone from overly wet conditions to the beginnings of a drought.

That is about all the room I have for this week. Let’s hope we see things dry out for those who need it, and that a little heat will move in.

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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