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Weather: The hottest Manitoba summers

There isn’t a clear-cut definition of what makes a heat wave

For the first time in a while, I am going off grid and enjoying the great outdoors. So, this time around I am digging back and refreshing an older article about the warmest Manitoba summers.

So far this year, using Winnipeg as a data point, we have the beginnings of a hot summer, but August needs to be really warm to make it one of the top five or 10 summers. While I may or may not have a forecast this week due to my location and lack of reliable internet, the medium-range models, as I write this, are pointing towards a very warm first half of August.

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Surprisingly, trying to find and pull out heat waves that have hit southern and central Manitoba over the years is a fairly hard thing to do. The difficult part is trying to determine how to define a heat wave. Is it two, three, four, or more days in a row above 30 C, or should it be above 32 C? What if you choose three or more days in a row above 30 C as the criteria and you have a year that sees two days in a row above 30 C then a 29 C day then a couple more days over 30 C, another day in the high 20s then back above 30 C? To me, that is a heat wave, but according to the criteria set out, it wouldn’t count.

When you have to look at over 100 years of data, you need to have some kind of criteria that will allow you to plow through the data fairly quickly and pick out years that have had some kind of heat wave. Trying to look through all the data manually becomes a very long and tedious pro- cess. I know, I’ve tried. Since coming up with a single criterion that would work well didn’t seem to be working out either, I had to come up with another way of picking out years that have seen significant heat waves.

The method I chose probably missed some small and possibly significant heat waves, but I decided that if a year had a large or long-lasting heat wave, then the mean or average temperatures for the summer would be abnormally high. With that in mind, I took another look at the data for Winnipeg (which goes back to 1873) and started looking for anomalously warm summers by looking at the average temperatures for June, July, and August, combined.

The first thing I identified was which years had really warm/hot summers. Now, the value I used was the mean June-to-August temperature, which is the average temperature for the entire summer season. For Winnipeg, over the whole 138 years of data, the mean temperature over the three summer months was 18.2 C. Just for fun I double-checked to see how all of the 138 values “fell” around this aver- age, and as it should be, the distribution was nearly a perfect bell curve. That is, there was an equal number of years warmer and cooler than 18.2 C and the majority of the years (about 88 recorded values) fell within 1 degree of the average, with the number of years quickly dropping off the further you went away from the average. For example, only five years recorded a mean summer temperature greater than 21 C and only two colder than 16 C.

From this, I decided that a mean summer temperature warmer than 20 C would be my threshold for what a hot summer would be. Going back in time, here are our warmest summers: 1988, 1983, 1981, 1961, 1959, 1955, 1947, 1937, 1930, 1929, 1921, 1919, 1910. Interestingly, there were no really hot summers in the late 1800s and while 1936 had a really hot July, the summer as a whole did not crack our list of the top hottest summers. Also, if you were born in the early ’60s you never would have experienced a really hot summer during your childhood as there were no really hot summers from 1962 all the way until 1981.

When I looked a little closer at these values I decided that cool or cold Junes would often skew the values a little bit, so I re-crunched the numbers this time only looking at July and August or what we in Manitoba typically refer to as our real summer months.

This time I bumped up the threshold to 21 C and ended up with four really, really hot years. 1930 had a hot summer with a mean temperature of 21.2 C, then came 1936 with a mean temperature of 21.7 C. We had to wait until 1955 for the next hot summer with a mean temperature of 21.5 C. Finally, the hottest summer of them all came during the early ’80s when back in 1983 the mean temperature for the months of July and August came in at a sweltering 22.3 C.

If I drop the July-to-August threshold down to 20 C then the number of hot summers jumps a fair bit. At left is a list of the hottest summers over the past 138 years, based on mean July and August temperatures. For these two main summer months the 1980s remain the hottest decade, with five years recording summer values greater than 20 C. For 2020 to become one of the top five hottest summers, we would need to see a mean monthly August temperature of 21.4 C, which just might be possible if the latest medium- to long-range models hold true.

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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