What is the right way to measure heatwaves


Europe: Extreme maximum temperature July 19 - 25, 2015, computer generated conours. Image: NOAA

by Sarah Perkins (from Sarah in Science blog)
Right now I’m on a working holiday (though far more on the work side!) in Europe. Also right now there is a rather large heatwave that has hit Spain, is currently over France, the southern U.K., Switzerland and Italy and is starting to move towards Germany, Austria and the Czech Republic (currently where I am).

The other day, the hottest ever temperature in London during July was recorded, and many locations in Spain and France have tipped over 40 (somewhat ironically the next leg of my journey is a climate conference in Paris).

I’ve also seen a bit of rubbish floating around the internet saying that Europe isn’t really in a heatwave and, frankly, that everyone should suck it up and enjoy the warm rays of sunshine.

So I thought this would be as good a time as ever to talk about how heatwaves are measured.

The U.K. Met Office uses this definition: at least 5 consecutive days where the temperature is at least 5oC above the daily maximum (get that?). {Edit - I originally reported this is what the World Meteorological Organisation uses, but I have been corrected. It is, however the definition in Wikipedia}. The threshold used is specific to each day of the year, so thresholds in spring are cooler than summer thresholds, and you can feasibly have a heatwave in winter too. It’s also relative to the location, so thresholds in London, say, will be cooler than Melbourne.

There are pros and cons to this definition. Let’s go with the pros first.

The definition is relative to location and time of year.  It’s worth keeping in mind that we’re all adapted to the climates we live in. The temperature in Český Krumlov today was around 28-30, to me this is lovely summer weather that I really enjoyed exploring this beautiful town in, but then I’m Australian (o.k., so today was more holiday than work…..). Many residents the Czech Republic did not seem to be coping so well. Smack on a few consecutive days under these conditions and it will wear them down. The same goes for any other living things in this climate – trees, pets, wild animals, crops.

The relativeness to the time of year is also a nice feature. We generally associate heatwaves as summer events since this is when the most disastrous impacts occur. Indeed, some abnormally warm weather in winter can be pretty enjoyable. But we’ve got to remember that such conditions are not normal to the time of year, and can disrupt the reproductive cycle of staple crops, and, as what’s currently happening back home, preventing some much anticipated snowfalls. 

Now let’s go with the cons.

Firstly, the definition stated above is antiquated. This group, who came up with the index about 15 years ago don’t actually use it anymore. They realized (as did I when I wrote this paper) that it doesn’t work everywhere (particularly the tropics), which is a bit redundant for an index that’s meant to be relative.

Secondly, 5 consecutive days is too long to be the minimum length of time a heatwave should last for. Over many locations in Australia, we rarely get heatwaves of over 5 days - due to the synoptic patterns that govern our weather -  yet we still definitely get heatwaves. The Australian Open heatwave in 2014, which received worldwide media coverage, is a perfect example of this.  Also, the impacts of heatwaves on human health, infrastructure, and our native ecosystems can kick in from 2 consecutive days. Particularly in the case for human health, this is a major issue if nighttime temperatures aren’t cool enough.

So, what’s the right way to measure heatwaves then?

Truth be told, there is no ONE way. In fact, there are literally hundreds of heatwave definitions. Sure, a lot of them are closely related, but how they are calculated is different. Some incorporate relative humidity. Some are a mixture of daily maximum and minimum temperatures. Some only measure summer events. Some used fixed thresholds (e.g. days above 300C). Believe me, the list goes on and on and on…..

Jee, I don’t even stick to a single definition! In a lot of my research activities, I use the Excess Heat Factor, the official definition of the Australian Bureau of Meteorology. This index is kinda neat, since it includes a measure of acclimatisation, as well as identifying how hot it is against the background climate. However on my website Scorcher, heatwaves are measured based on at least 3 consecutive days exceeding the daily 90th percentile (each day is in the hottest 10% for that day of the year), since this was easier to calculate and show on graphs. All in all, the events measured by these definitions are pretty much the same, though they aren’t 100% identical.

Given the vast range of impacts heatwaves have, I don’t think we’ll ever fully agree on one definition. Sure we might (and should!) be able to narrow it down, but I think a complete one-size-fits-all approach is a little out of reach.

But I certainly do think it’s time that the guidance on a formal definition is changed. 5 days is certainly too long for a minimum length. And a threshold that does not work everywhere, particularly in regions where impacts can potentially be very high, needs quick attention. Based on the current state of research, it really is time for more formal guidance, perhaps by something like the World Meteorological Organisation, ton an overarching definition. That way, weather and climate services the world over can adopt their own heatwave definitions that are more in line with current scientific research.

Putting all of the above aside, I do hope everyone who is in the thick of the hot European weather is taking care. If it is 1, 3, 5, or even more hot days in a row, if the temperature is 30C, 35C or 40C, I hope you’re taking the appropriate measures to keep cool, regardless if a heatwave has been officially declared.  Remember to keep yourselves hydrated, avoid any strenuous exercise or activities, and stay out of the sun. Don't forget to look after those around you too.

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