Humid heat sucks. This is how we measure it.

Bondi Beach, by Rabs003 (

When a heatwave hits, all anyone wants to do is compare scores. “It’s 38 °C in Sydney? Well, it’s 44 °C over here!”

Leaving aside that people in different cities respond to heatwaves differently (not just behaviourally but physiologically—our bodies change with repeated exposure!), remember that the 38 or so degrees Sydney is experiencing right now is not like the 44 you’d feel in Alice. As Darwin residents will tell you, humidity is an important part of heat stress.

Our bodies are pretty fussy about temperature. There’s a bit of variation (by person, time of day and other factors), but once we start creeping past 38 or 39 °C internally, we have problems. Real, life-threatening problems.

We sweat to stop that from happening, and evaporated sweat accounts for most of our heat loss on warm days. But when it’s too humid out, our sweat drips off without evaporating, and it doesn’t cool us down. And then we get hot inside.

Scientists like to put numbers on things, and one of the many, many ways to measure the heat/humidity/radiation combo is called the Wet Bulb Globe Temperature (WBGT). WBGT is a favourite of sports organisations, unions, researchers and others for use in extreme heat policy: when the WBGT gets too high, people start calling matches and work off.

The WBGT usually looks a little lower than the regular (dry) temperature, so it can be easy to get complacent about it. For example, the US Army’s guidelines on heat stress recommend breaks every hour based on the WBGT. The first restrictions—the ‘yellow’ rating—kick in at a WBGT of 29.4 °C. The highest, or ‘black’, rating recommends hourly breaks for even light work when the WBGT hits 32.2 °C.

And this is for people healthy enough to be in the army. People have varying tolerances to heat stress, and the young, the sick and the elderly are particularly vulnerable.

Now let’s look at the WBGT at Sydney stations right now (well, 5 PM on Saturday). Going left-to-right, we have temperature, WBGT in the sun, and WBGT in the shade.

Even if you’re in the shade, Sydney Olympic Park is nudging the black rating. All but two stations are at least yellow. If you’re out in the sun—and I really hope you aren’t—every single station is in the black.

The takeaway is this: you should not be doing any exercise in this heat. You should not be in the sun continuously.

Apart from drinking plenty of water, there are things you can do to avoid heat stress. But here’s one last thing: heat stress isn’t flashy. It mostly (but not only) hits vulnerable people, and it hits fairly quietly. Many people die in isolation. On days like this, check in with your folks and your neighbours. They’re going to happen much more often in the future.

(* Badgerys Creek looked a little suspiciously high yesterday. Might’ve been a station error, but IDK.)

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