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The difference between heat waves and hot spells

Mid-June this year has definitely met the temperature criteria for a heat wave

It has been pretty hot across a large portion of the Prairies over the last month or so. After a cold start to April, temperatures began to heat up, with May coming in well above average across all three Prairie provinces. The recorded daily temperatures during May and until June 24 show most locations have seen more days above 30 C so far this year than all of last summer put together. A little unusual this year is where the hottest temperatures have been: across the north!

All this heat, and all the talk about heat warnings and such, got me thinking back to the last time I looked at the topic of heat, and in particular, heat waves or hot spells. It’s been a while, so I thought we should revisit this topic. First, how do you characterize a heat wave? Well, 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. Long-term normals for mid-June across the Prairies range from around 22 C in Alberta to around 25 C in Saskatchewan and Manitoba, so with highs in the upper 20s to low 30s, I would say that we’ve definitely experienced this first heat wave characteristic.

Now, one hot day does not a heat wave make, so our second characteristic is duration. Again, after a little digging, it appears most places set the duration for a heat wave at two or more days. So far this year, most areas that I checked have seen a couple of periods of excessive heat that have lasted from three to six days, so we can check this characteristic off the list too.

Our next characteristic is humidity. This one is a little tougher to define, as you can have heat waves under lower or higher humidity, but generally, when really epic or intense heat waves are discussed, they also include high levels of humidity. This makes sense, as hot temperatures plus moisture in the air makes the air feel even hotter; 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. Across the Prairies, early-season heat waves are often dry, with plant growth just getting going. As we work our way into summer, the humidity tends to build, as some areas began to experience in mid-June.

Contents under pressure

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, in 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 the air aloft — 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 toward the ground and will sink. As this air sinks it will get compressed, and if you’ve 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 hot 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! Luckily for us, we are currently sitting on the northern edge of a large ridge of high pressure centred over the eastern two-thirds of the United States. This looks like it will allow moisture to occasionally make its way into our region in the form of thunderstorms. Unfortunately, thunderstorms can be very hit and miss, bringing too much rain to some areas and none to others.

That is about all the room I have for this week. Let’s hope things cool down just a little bit over the next while and that you do get just enough rain.

About the author

Co-operator contributor

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

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