By Angus Dalton
It’s not the heat that gets you … and it may not exactly be the humidity, either.
There’s another under-appreciated measure of how muggy a day will get and it’s far better at indicating how sapped and sweaty you’ll feel upon stepping outside: the dew point temperature.
Dew point temperature is a far better measure of how much water is in the air and how muggy a day might feel.Credit: Getty
Scrutinising the relative humidity might give you an idea of whether your hair will frizz or the risk of your freshly baked pavlova collapsing on a hot day, but meteorologists prefer dew point temperature to get a read on how uncomfortable a humid day will actually feel.
Taking Sydney’s Boxing Day conditions as an example, the dew point temperature is 20 degrees, 5 above average and in uncomfortably muggy territory, but by midday the relative humidity will only be about 60 per cent.
That’s because relative humidity is shackled to temperatures, whereas the dew point is a direct measure of how much water vapor saturates the surrounding air. It’s otherwise known as “absolute humidity” for this reason.
You could also think of the dew point temperature as the “feels like” temperature of humidity.
“Basically, the dew point is the temperature at which the air will reach saturation,” the Bureau of Meteorology’s Dean Narramore said. “If you see a dew point of 20 degrees, that means if things cools down to 20, you would reach saturation.
“It’s a much better indicator of how humid or how moist an air mass is.”
Relative humidity, on the other hand, is the percentage of moisture in the air compared to how much moisture the atmosphere could be holding. The hotter the air, the more water vapour it can hold – about 7 per cent more water per degree of temperature.
For this reason, relative humidity fluctuates with temperature, sometimes plunging from 100 per cent to 50 per cent over the course of a morning as the day heats up, whereas the dew point temperature tends to remain steady.
“I wish we talked about it more, to be honest, because it’s not dependent on anything, it’s explicitly just showing you how moist the atmosphere is,” Narramore said.
Narramore refers to a hot and extremely muggy Sydney day in December, Tuesday 19, as another example.
“Everyone was complaining how disgusting it was outside, but the relative humidity was only 60.”
At 1.30pm at Observatory Hill that day, the relative humidity was 64 per cent, but the dew point temperature hovered just below an oppressive 24 degrees, so the city felt swamped with muggy air.
Dew point temperatures in the mid-20s are rare, Narramore said.
“That’s when it starts getting really hard to sleep and feeling like you could go outside and cut the air with a knife. It’s really thick, heavy air.”
On humid days, it’s harder for sweat to evaporate – our body’s main cooling system – because the air surrounding the skin is already packed with water. That’s why dew point temperatures at and above 20 start to feel stifling.
“On rare occasions Sydney can get up to 25, 26. The tropical areas are generally always in the 20s. The world record is in the low 30s. Your sweat couldn’t even evaporate with that kind of air mass.”
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