As soon as summer temperatures rise above the norm for a few days the media start getting excited about heat waves. Sometimes it just makes for a catchy headline but there’s no doubt that some are real episodes of extreme heat that can have serious consequences. So what is a heat wave, and why should we worry about them?

There’s no strict definition of a heat wave. It really is all about unusually hot and/or humid weather, and temperatures that aren’t usual for the area in question. In some countries it has to last three days, in others five. Some specify that the temperature has to be a certain number of degrees above the recorded norm.  In Adelaide, Australia, it’s three days over 35ºC or five over 40. But in Sweden five days of the weather exceeding 25 ºC meets the definition.

A heat wave isn’t about absolutes. It concerns the discomfort that people experience. It also ties in with the effects, and the coping mechanisms that homes, cities and environments have – or don’t have – to cope with the unusual weather. But what causes it?

Specific causes of heat waves come about in places with different topographies, environments and wind patterns, proximity to the jetstream and other global meteorological factors. The common factor is typically a high pressure system that traps hot air over a given location for days or even weeks at a time, potentially causing extreme heat.

The source of human discomfort is also variable. High temperatures alone can be the problem in some places. In others the temperatures may be lower but unaccustomed high humidity and stickiness is what gets people down. Some parts of the world are afflicted by foehn (or föhn) winds that occur on the leeside of mountain ranges, in the rain shadow zone. These are dry winds but they can cause oppressively hot conditions.

It seems that heat waves have always been with us, but there is also growing evidence that a recorded increase in extreme weather of all kinds is a by-product of climate change and global warming. Heat waves don’t always receive the same media attention as floods, storms, blizzards and unseasonal weather, but that doesn’t mean they aren’t hugely destructive. In fact, heat waves are documented as causing more deaths than storms, floods, hurricanes and tornadoes combined. Extreme heat can also exacerbate the effects of environmental pollution, increasing the risks of respiratory problems, as well as hypothermia and heat stroke.

The impact of heat waves ties into a vicious circle of over-exploitation of resources that are already implicated in climate change. Those who can afford to crank up the energy-guzzling aircon units. Consumption of water – already a scarce resource in many areas experiencing drier climates in recent times – goes up. Poorer communities and food production in marginal areas can be heavily hit by just a week or two of extreme heat or humidity.  Heat waves turn out to be more than a few sweaty days and, if climate change is not dealt with, we can expect more in years to come.

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