The Great Heatwave of 2021

Is it reasonable to call the heatwave which lasted from June 26th through July 2nd (or maybe even June 25th through July 3rd) "unprecedented?"

(if you are reading this in the distant future some of the dashboards below may have updated to reflect new data or formatting, or they may be broken, or they may no longer make sense. To see the original images for this post refer to this twitter thread.)

Let's start with a quick list of what it didn't and didn't do:

  • 8 days in-a-row hitting 29°C, beating 6 days from June 1961.
  • 7 days in-a-row at 30°C, beating 6 from June 1961 again.
  • 4 days in-a-row at 34°C, beating 3 from July 1924.
  • 3 days in-a-row at 35°C, tying July 1924 again.
  • 2 days in-a-row at 36°C, which had never happened before. Since 1880 Edmonton has only had 4 days reach 36, and 2 of them were June 29th & 30th, 2021.

It also set new daily records on June 28th at 34.2°C, and on June 30th at 37.0°C. 2 new records might not sound like a lot, but this heatwave overlapped with two previous heatwaves in 1924 and 1937:

In Edmonton's list of hottest days the week of June 29 through July 3 holds 7 of the spots, from 3 different years:
  • At the very top June 29, 1937 remains as Edmonton's hottest day on record at 37.2°C. 
  • 2021 has 3: #2 37.0°C on June 30, #4 36.2°C on June 29, and #7 35.2°C on July 1.
  • 1924 has 3: #3 36.7°C on July 2, #5 35.6°C on July 1, and #8 35.0°C on July 3.

Many of the temperatures this week would have broken daily records during any other week of the year, but they weren't hotter than the 1924 & 1937 heatwaves. And so because of that 2021 ends up with "only" 2 new daily records.

That is just for the daily High temperatures, though. The daily Lows really were unprecedented:
  • Possibly 10 overnight Lows in-a-row of 15°C, tying August 1984.
  • 9 Lows in-a-row of 17°C, beating 6 from August 2020. (the record before that was 4)
  • Possibly 6 Lows in-a-row of 19°C, beating 2 from July 1975
  • 3 Lows in-a-row of 20°C. We had never had more than 1 at a time before.
The heatwave set 8 new daily records for overnight Lows. Almost every day from June 25th through July 3rd has a new record, with June 27th as the only day that missed.

2021 has 3 of Edmonton's top-10 warmest Lows ever recorded:
  • June 29th is in 8th spot at 20.3°C.
  • June 30th set a new record for all-time warmest Low at 22.8°C
  • And then the next day July 1st set a new-new record for all-time warmest Low at 23.4°C
So overall, for the High temperatures the Great Heatwave of 2021 was a bit of a combination of previous heatwaves: the super-hot temperatures of June 1924 & 1937, with the long duration of June 1961. For the Lows though, we really hadn't seen anything like it.

Enough words. Now it's time for some charts.
Daily Highs
Here we have the High temperatures for the summer of 2021, with the heatwave right in the middle at the end of June. Our average Highs at that point of the year are about 23°C, so the heatwave days were 7~14°C above average. There were a lot of 30°C days packed in there, but only 2 records were broken because the old records were some of Edmonton's hottest days ever recorded.

Daily Lows
Here are the Low temperatures, with all of the new records set during the heatwave. The average Lows at that point of the year are about 12°C, so these records were 5~11°C above average. Earlier in the year there are also some other daily Low records which were broken.

While Edmonton was having its semi~unprecedented heatwave something similar was happening in Calgary and Vancouver. So for a lot of the charts today those cities will be included as well.

30°C Highs In-a-Row
For 30°C Highs in-a-row Vancouver tied its previous 4 day record from 1911. In between those 110 years they also had a 3 day streak in 2009. Edmonton's 7 days in-a-row beat the previous record of 6 days in 1961. Recently we had gone 3 days in 2015 and 2012, and 4 days in 2008 and 2002. Calgary's 7 days did not catch their record of 8 days from 1917, but it did tie for 2nd place with 1896. In recent years they had gone 4 days in 2002 and 5 in 1996.

35°C Highs In-a-Row
For 35°C Highs in-a-row Edmonton tied the 1924 heatwave with 3. And the only other times when we have hit 35°C were 1933 and 1937. Calgary's 3 days beats their previous 2 day streaks in 1919 and 1933.

Hottest Days Ever Recorded
For this heatwave we saw that Edmonton had 3 days in the top-10: #2, #4 and #7. 

Vancouver had its #7 and #8 hottest days, but did not get as hot as its July 2009 heatwave at 34°C. 

And Calgary had 3 days in its top-10: June 29th and July 1st are tied for 2nd hottest day overall at 36.3°C, and June 30th was #6 at 35.9°C.

30°C Highs In-a-Row Around Edmonton
The heatwave didn't just hit the city of Edmonton. For nearby stations the International Airport was at the low end of things with only 5 days in-a-row at 30°C, but most of the other stations had 6 or 7 days. In this chart it's also interesting to see that the 1961 heatwave showed up at several of the stations.

35°C Highs In-a-Row Around Edmonton
Through a lot of this Namao was actually the hotspot, with 4 35°C days in-a-row, including 37.5°C on June 30th. And a little bit further northwest Campsise hit 37.2°C on June 29th and 37.4°C on June 30th.

16°C Lows In-a-Row
Overnight Lows aren't as exciting as Record! Breaking! Highs! but during this heatwave Calgary had 6 nights in-a-row above 16°C, when their previous record was 2. Edmonton set a new record with 9. And Vancouver's 8 Lows was their 2nd most, but was not enough to catch the 10 in 2009.

17°C Lows In-a-Row
For warmer Lows at 17°C all three cities set records. Calgary had 3 in-a-row, when before it had only ever had 1. Vancouver had 7, beating 6 from 2009. And Edmonton has had at least 9, beating 6 from 2020.

19°C Lows In-a-Row
And taking the Lows one step further to 19°C Vancouver had 3 in-a-row, which tied 2009, 1998 and 1941. Edmonton had a new record with 6, beating 2 from 1975.

Edmonton also had 3 Lows in-a-row of 20°C, when previously the most had been 1. Vancouver had 2, but back in 1941 they had 3 in-a-row.

Hottest Nights Ever Recorded
For the overnight Lows in Edmonton 2021 set a new #1 record of 23.4°C, and also has the #2 and #8 spots. Calgary has a 4-way tie for their top spot at 18.3°C, including June 29, 2021. This heatwave also had their #5 and #8 spots. And Vancouver didn't get close to their all-time record of 22.4°C from 2009, but did have the #3 and #4 spots.

Lows Around Edmonton
The overnight Lows for the outlying areas around Edmonton were not as warm as they were in town. But some of the stations had a few very warm nights.

High Temperature Records
In this chart each of the bubbles is a daily High temperature record. This heatwave only set 2 new records for daily High temperatures, and they are over on the right side.

Low Temperature Records
In this chart each of the bubbles is a daily Low temperature record. So far 2021 has set 11 new records, including 8 during this heatwave, and the all-time warmest record Low of 23.8°C on July 1, 2021.

And so that brings us to the end of our lookback at the Great Heatwave of June-July 2021. The Highs were as hot as anything we had seen before, and they were hotter for longer. The Lows were even more unusual, setting all sorts of records and giving us a solid week of unusually hot nights.

If you have some extra time Why Heatwaves Are Not the Whole Story is a good follow up to this look at the heatwave of 2021. This was Edmonton's first really big heatwave in a long time, but there has also been other warming going on in the background too.

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