Before we begin I must first emphasize that any discussion about the temperature refers only to official readings that were recorded inside a Stevenson screen or the modern equivalent, and placed away from asphalt and buildings. Invariably when the discussion comes up that very few places in Canada ever reach 40°C (104°F), someone will pipe up about how they record temperatures well over 40 degrees quite often at their place. This is because a backyard thermometer can exaggerate the air temperature by as much as 10 degrees, and even more if you like to turn your mercury filled glass thermometer into a greenhouse by placing it directly in the sun. When we try to compare temperatures at the water cooler to see whose places was hottest yesterday, the guy with the worst location wins most of the time.
Now that we have cleared that up, let’s move on to defining an extreme heatwave: the hottest temperature recorded during a heatwave. Therefore, the most extreme heatwave is not necessarily the longest heatwave, nor is it necessarily during a hot summer. That is why the hottest month, the hottest summer, and most extreme heatwave all occurred in different years (1906, 1958, and 1941 if you must know).
1998 is considered the gold standard of heat by virtue of the fact that it slides in near the top in all three categories just mentioned. That is in addition to being the warmest year on record both in British Columbia and globally (according to satellite data).
We will then for the purpose of this exercise run a comparison between 1998 and the 15 most extreme heatwaves on record. The bars on Figure 1 represent the average temperature difference in degrees Celsius between the 1998 heatwave and the other years in question.
The 2.4 degree difference between 1941 and 1998 means that the average weather station common to both years recorded a maximum temperature of 2.4°C (4.3°F) higher in 1941 than 1998. This is an average of all stations in common, including Atlin which was 7 degrees cooler in 1941.
Nothing comes close to 1941, during which three different places recorded a temperature of 112°F (44.4°C) – Lillooet, Lytton, and Barriere. Many years don’t see a temperature of 104°F (40°C), and typically when it happens there will only be a couple of stations. No so with 1941 where more than 1/4 of all stations across BC recorded a temperature of 104°F (40°C) or more, and many of those were well in excess of that benchmark.
Let’s compare the extreme hotspots within the province, typically found in the southwestern interior. In 1941 Lillooet was 112°F (44.4°C), and only 105°F (40.4°C) in 1998. Oliver was 111°F (43.9°C) in 1941, but only 104°F (40.0°C) in 1998.
We find similar results with the other years in question. Lytton was the hottest location in 2007 at 106°F (40.9°C), which is a far cry from the 112°F (44.4°C) recorded in 1941.
2009 was one of those rare years where the coastal communities could match the interior degree for degree. The Vancouver airport set the all time record that month, and Bella Coola and Gold River, both of which are located at the end of fjords, were over 41 degrees Celsius. This was similar to 1941 for many coastal communities, but the interior was a different story. Lillooet was the interior hotspot at 106°F (41.2°C), and far short of 1941.
Note that I am referring to the Fahrenheit first because temperatures were recorded to the nearest °F prior to the 1980s. Today’s weather stations either record to the nearest 0.5 °C at manual stations or the nearest 0.1°C at automated stations.
Moving on down the list, the hottest spot in July 0f 1961 was Lytton at a mere 104°F (40.0°C), which was 8 degrees cooler than the temperature recorded 20 years earlier.
July of 2006 was certainly one of the hottest spells on record in the southern interior, but still fell short. Lytton was the hot spot at 108°F (42.1°C).
We would be remiss without mentioning the most recent heatwaves from 2015 when the province experienced not just one, but two extreme heatwaves. The first was in June, weighing at 18th spot, this was the most extreme heatwave to ever strike the province during the month of June. 40 degrees is extremely uncommon in June, but several places managed to achieve that beat in 2015, including Hedley at 42°C (the hottest temperature ever recorded in June), Warfield at 41.1°C, Osoyoos at 40.9°C, Ashcroft at 40.6°C, and Revelstoke at 40.1°C. Some of the places like Revelstoke were rivaling 1941 temperatures.
July was even hotter province-wide because the heat was more widespread (the water is too cool in June to maintain widespread heat on the coast); July 2015 was the 10th most extreme heatwave to ever strike the province.
Outside of 1941, there have only been a handful of weather stations recording more than 108°F (42°C) like Lytton did in 2006, and most of those were due to instrument error or improper site location. Osoyoos is one such place that recorded at (109°F) 42.8°C in 1998 while the other Osoyoos weather station a block away didn’t even reach 40°C. Similarly, Kamloops recorded a 108°F (42.2°C) reading in 1975 while the nearby station was not recording anything particularly unusual.
The one place that has exceeded 42 degrees multiple times outside of 1941 is Spences Bridge, which can only be classified as the hottest place in BC during the summer months. Weather records in Spences Bridge were not around in 1941, but they were in existence in 1906, 1908, 1994 and 1998 when the temperature climbed above 42 degrees Celsius. How hot Spences Bridge was in 1941 during British Columbia’s hottest day is anyone’s guess, but 45°C (113°F ) is not out of the question.