2015, the Hottest Summer on Record?


Parched Chilcotin grasslands in 2015.

The summer of 2015 has been hot, but how does it stack up against past summers?
Meteorologists define summer as June, July, and August, and of no surprise to anyone living in British Columbia, the summer of 2015 enters the records books as the hottest on record – at least on the province’s south coast.

An earlier write-up discussed the hottest day on record (July 16th, 1941), but that extreme heat wave did not lead to the hottest summer. That’s because the hottest day is the one with the high maximum temperature while the hottest summer is the one that yields the highest average mean temperature over the entire season.

The mean temperature is the average between the high during the day and the low at night. As a result, the numerical value ends up being a lot cooler than most people envision. If you were to ask the typical person on the street to estimate the average temperature over this past summer they’d probably say something like 30°C (86°F). Maybe even hotter. That’s because they are inclined to think of (and experience) the afternoon heat when it’s 30°C (86°F)  a lot more than the morning coolness when it’s only  12°C (54°F). The average of these two numbers is only  21°C (70°F), and yet surprisingly, is hotter than almost anywhere in Canada. Only southern Ontario and Spences Bridge, BC have averages that hot.

You are thinking to yourself that this cannot be right because we get two months hotter than 21°C most of the time. Ah, but you’re forgetting about June, which is much cooler than the other two summer months (unless you live in the north where August is the coolest summer month). Of course, you’d be forgiven for forgetting this fact since June was every bit as hot as August this year, and August was a lot hotter than normal.

Even though Spences Bridge has the hottest summers in British Columbia, it doesn’t hold the record for the hottest summer ever recorded because the town’s weather station no longer exists, nor did it exist for most of the 20th century. Therefore, the honour for the hottest summer ever recorded belongs to Lillooet from 1958 when the average summer temperature was 23.8°C (74.8°F). For what it’s worth, this was achieved with an average daily maximum of 32.2°C (90.0°F) and an average daily low of 15.3°C (59.5°F). Even with the slew of hot summers over the past decade and a half, no summer has been able to come close to matching the 1958 record — that is, until 2015.

The hottest areas of BC are the Fraser and Thompson canyons as well as the south Okanagan valley, and that fact is reflected in the list of the 10 hottest summers ever recorded in the province

  1. Lillooet at 23.8°C (1958)
  2. Ashcroft at 23.6°C 2015)
  3. Oliver at 23.6 (1958)
  4. Lillooet at 23.5°C (2015)
  5. Lytton at 23.4°C (2015)
  6. Lytton at 23.4 (1958)
  7. Ashcroft at 23.4 (1958)
  8. Osoyoos at 23.3°C (2015)
  9. Spences Bridge at 23.3°C (1998)
  10. Lillooet at 23.3°C (2009)

Now, as the list above implies, 2015 was slightly cooler than 1958, and this trend extends elsewhere across the southern interior:

  • Kamloops: 2015 averaged 22.1°C, weighing in as the 4th warmest summer, just 0.1°C shy of 2004 and 2009, and 0.2°C behind the all time record from 1958 (22.3°C).
  • Penticton: 2015 was the second hottest summer at 21.6°C; only 1958 was hotter with an average of 21.7°C.
  • Vernon: 2015 averaged 21.0°C, good for the third hottest summer, behind 1998 (21.1°C), and 1958 (21.5°C).
  • Cranbrook: 2015 was the second hottest summer on record at 19.1°C; only 2003 was hotter at 19.5°C.
  • Tatlayoko: 2015 averaged 15.0°C, tying 2014, 2009, 2004, and 1961 for the hottest summer on record.

Coastal British Columbia was an entirely different matter, with weather stations shattering their 1958 record. Here are two of the longest running weather stations in the province:

  • Agassiz: Beat 1958 by 0.5°C with an average of 20.4°C.
  • Shawnigan Lake: Beat 1958 by 0.4 degrees Celsius with an average of 19.3°C.
  • Victoria: Beat 1958 by 0.1 degrees Celsius with an average of 16.9°C.

At the end of the day, the summer of 2015 was the hottest on record in coastal BC, and close to that in the southern interior. Province wide, 2015 was the second hottest on record while 1958 still stands out as the hottest summer on record.


Figure 1: Summer temperature deviation from 1895 to 2015.

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4 Responses to 2015, the Hottest Summer on Record?

  1. Nanaimo 2015 was not warmer than 1958 using daily TMAX data from Environment Canada data.

    How does 2015 stack up if you use Tmax instead of Tavg?

    • That’s quite interesting. I get an even larger increase using Tmax on the coast. Agassiz was 0.8°C warmer in 2015 than it was in 1958 when the old record was set; Shawnigan Lake was 0.5°C warmer in 2015.

      In the interior, Kamloops was 0.9°C cooler in 2015, and in fact, 2015 only ranks 5th by Tmax. Pentcton was only 0.1°C cooler in 2015, good enough for 2nd behind 1958.

      Tatlayoko Lake was a full 2°C warmer in 2015, but still only good enough for 3rd spot all time (1958 was only the 21st warmest summer). 2009 and 2014 were the top two using Tmax.

    • I was using the “homogenized and adjusted data” for Agassiz (http://www.ec.gc.ca/dccha-ahccd/), which pushes 1898 and 1906 down below 2015 and 1958. The reason some weather stations have adjustments is because stations moved. Agassiz CDA either moved in 1924 or the method used to measure temperature changed. The cynic might say the numbers were adjusted prior to 1924 to make Global Warming look bigger, but I think the the adjusted numbers were made for scientific reasons to more accurately compare the past with the present.

      I’m not big on using a single station or location though because stations move and instruments fail. That’s why for the graph above I used all weather stations (unadjusted), and assumed that the few that were in error would cancel others in error the opposite direction, or at least were overpowered by the 100s of accurate stations.

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