Relatively Extreme: Heat Waves in NL

At the risk of stating the obvious: It’s been hot in Newfoundland this summer. This july we broke more than a few temperature-related records, gaining the attention of both local and national media. The phrase we kept hearing last month: ‘heat wave’. But whether that term fits depends who you ask.

Heat waves are one of the most confusing weather events meteorologists track, mostly because there seem to be as many definitions for heat waves as there are meteorologists. Criteria vary dramatically between organizations, countries, and even within countries (the U.S. alone has several definitions), but all require

  • temperatures to rise above a set level
  • for a minimum number of consecutive days

By Environment Canada’s definition (highs at or above 32oC for at least 3 days), heat waves are extremely rare in Newfoundland. For example, St. John’s hasn’t officially hit that 32oC mark once, let alone for three days. But this is a geographically limited definition, suitable only for warmer parts of the country. It’s useful for identifying events that pose serious health hazards, but will miss many locally significant events. And certainly this July felt like a heat wave: a long stretch of unusually high temperatures (unusual for us, at least).   That character of ‘unusual’ has to count for something, right? Fortunately, the World Meteorological Organization (WMO) agrees, and offers a more flexible definition based on local climatology. To the WMO, a heat wave occurs when

  • daily maximum temperatures exceed local climatology by at least 5oC
  • for a minimum of 5 consecutive days

Individual heat waves are separated by at least one day that does not meet the temperature requirement. By this definition, heat waves can occur anywhere, but will always be an extreme event – locally rare, with the potential to stress the surrounding environment. We can consider these long ‘hot spells’, a broader family of events that meet the temperature criteria of a heat wave, but without any consideration of duration.

Applying the WMO’s definitions to St. John’s can help us put this July in context and estimate how unusual it was. First, we need to select a threshold; on average, the daily summer high in St. John’s is 18.7oC. By WMO standards, hot spells (and heat waves) should then exceed 23.7oC; let’s round this to 24oC. Using this definition, a typical St. John’s summer will experience 8.5 hot spells. Heat waves are much less common; on average, only one occurs every 2.7 years.

So what about this year? To date in 2014, St. John’s has had six hot spells and (drumroll)… four heat waves. All began in July, and the last ended on August 5th. In climatological terms, that’s more than we’d typically expect in a decade

So is this climate change?
Whenever we see something like our record breaking sequence of July heat waves, we need to reconsider our definition of ‘normal’ climatology. How well does this unusual series of rare events fit with our past climate? With a little statistics (click here if you really want to see the details), we can address a few relevant questions:

Q: How unusual is it to have 4 heat waves?
A: Unusual, but not impossible. We’d expect four or more heat waves roughly once a century. Other years have seen multiple heat waves; for example, three occurred in 1984, and two occurred in 1998, 2003, 2004, and 2008.

HW_count

Q: Were these events usually long?
A: Nope. This year we’ve had two 5 day and two 6 day heat waves. St. John’s has had much longer events, including one in 2003 that lasted 13 days.

HW_Max

Q: Were these events unusually warm?
A: Again, no. The warmest event had a peak temperature of 29.5oC; we’d expect one heat wave in forty to hit these temperatures.  As for breaking temperature records: this is still two degrees below St. John’s all time high of 31.5. (Note: we did, however, break humidex records, which accounts for the impacts of humidity on perceived heat.  But that’s a whole other mess to sort through…)

HW_Duration

So: is this July a sign of climate change? Alone, no – within the context of our past climate, this July falls firmly into the category of ‘novel, but not unexpected’. But there are indications that these are becoming more common; of the 27 heat waves that have occurred between 1950-2014, 41% happened on or after 1998.   Years with multiple heat waves are apparently more common (it’s happened 4 times since 1998, but only once from 1950-1997). Altogether, this suggests that the past climate record may no longer reflect our present. While it’s still too early to say what this means for our future, it tells us we should keep an eye on our thermometers in coming years.

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