Summer in the UK is puzzling to watch from Australia. When 25C triggers a run on the beaches, and 28C triggers a heatwave warning, it can seem like the UK is overreacting a bit. But last week the UK got genuinely hot, with a warm-spell peaking at a record-breaking 36.7C. Still, the Australian media reacted with hilarity. This Buzzfeed Australia article is typical ("That's not a heatwave. This is a heatwave").

After all, Australia's temperature extremes are much hotter. Melbourne's hottest ever temperature, for instance, was 47C. A 2014 heatwave saw temperatures above 40C for four days. 

So should the UK just harden up? 

Temperature is relative. One person's hot is another person's mild. This holds true for heatwaves. Extreme summer in London may not be extreme heat in Melbourne. But wherever they happen, these events are dangerous. People die. They are among the most dangerous natural events. 

Heatwaves are generally defined as periods of unusually hot temperatures. The excellent Scorcher website that tracks current heatwaves in Australia defines heatwaves as "at least three consecutive days where the daily maximum temperature is in the top 10% of warmest temperatures for that calendar date". Interestingly, this also shows that heatwaves happen in winter.

Whenever they happen, heatwaves are getting hotter, longer and more frequent, and are forecast to keep doing so thanks to climate change. In fact analysts have already found that the latest hot weather in Europe was up to four times more likely thanks to climate change. 

The relative difference between heatwaves is reflected in heat warning systems. Heat warning systems are relatively recent: the first was developed after Chicago's 1995 heatwave. Different places have different thresholds for triggering a warning. For instance, in Melbourne a heat health alert is triggered by an average temperature of 30C. This is the average of the maximum daily temperature and the minimum temperature the following night, so could be triggered by a day of 40C followed by a night of 20C.

In London though heat health warnings start when day time temperatures reach 30C followed by nights above 18C. 

There's a lot of thought that goes into these heat thresholds. In fact the World Meteorological and World Health organisations have just released global guidelines for heat alerts. Some of the difference can be explained by what people are used to. Hotter places have higher thresholds that colder places (although experts aren't sure if this is true everywhere). People in hot places might know how to keep cool, or have lifestyles to suit the hotter weather, whereas people elsewhere might be caught by surprise. 

Heat health expert Liz Hanna wrote on The Conversation that the European heatwave of 2003 likely killed so many people (in the tens of thousands) because people didn't get a chance to acclimatise to the hot temperatures. 

"Humans can acclimatise to heat after five or six weeks of consistent exposure," she wrote. 

However climate change will present new heat extremes that we've never encountered before, and in Australia, our major cities are not well suited to these new extremes. 

Picture from { pranav }/Flickr licensed under CC BY 2.0

 

 

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