Several years ago, I took the time to analyze summer temperatures to see when the hottest summer occurred. With the summerlike heat we are beginning to see, I thought it would be interesting to take another look at that data. 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 process; 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, its 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 average, 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 C 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 and 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 (see table at bottom).
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-August threshold down to 20 C, the number of hot summers jumps a fair bit. Here 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. With summer heat building into our region early this year, the question starting to be whispered is, “Will we see record-breaking heat this summer?” As usual, only time will tell, as we all know just how unpredictable our weather can be at times!