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Anatomy Of A Heat Wave

It’s been a while since we’ve seen temperatures like we have over the last week or so. Going back through some of the weather records, the last time we saw temperatures this warm was back in July of 2007. With the forecast showing warm to hot conditions lasting all the way through to the end of the month, it is looking more and more like this July is going to end up well above average. The last time we saw that was back in 2003.

Now, I know I have been promising to do an article about all the rain that fell this spring and it is still in the works, but to be totally honest, I am away camping right now and all my rainfall data is sitting at home. So, as I sit here sweating away, trying to think of something to write about, the one topic that keeps popping into my mind is – heat waves.

First of all, 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 southern and central Manitoba for this time of the year are around 26 C, so with highs in the low to mid-30s, I would say that we are hitting 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 that most places set the duration for a heat wave at two or more days. If the current forecast holds true, we’ll be seeing around four to six days in a row with excessive heat, 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 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 levels of humidity. This makes sense, as hot temperatures plus moisture in the air, makes 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. So far, during this heat wave, it has been fairly humid, so we can check this characteristic off as well.

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 that is 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 atmo-sphere. 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 evaporat ing 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.


Withtheforecastshowingwarmtohotconditions lastingallthewaythroughtotheendofthe month,itislookingmoreandmorelikethis Julyisgoingtoendupwellaboveaverage.

About the author

Co-operator contributor

Daniel Bezte

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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