How humid is it? 3 things to keep you cool in a hot and sticky summer (and 3 things that won't)

La Niña has resulted in a humid summer for much of eastern Australia.

This climate pattern typically causes warm, humid air to rise from the western Pacific Ocean, ultimately leading to greater cloud formation, increased rainfall, and higher humidity along Australia’s eastern seaboard and tropical north.

So what’s the best way to cope with what’s left of a hot and humid La Niña summer?

Read more: Back so soon, La Niña? Here's why we're copping two soggy summers in a row

In praise of sweat (and evaporation)

Sweating (and the heat loss that goes with it) is by far the most effective way our bodies use to cool us down. But it’s not sweating alone that does the trick. It’s the evaporation of sweat that is key.

For every gram of sweat you evaporate, a little over 2.4 kilojoules of heat energy is taken with it.

That’s a bit difficult to visualise. So let’s think of it in terms of boiling water.

Evaporating roughly 140 grams of sweat results in enough energy (heat) “loss” to bring a litre of 20℃ water to boiling point (at sea level).

We were treated to some interesting facts during this year’s notoriously sweaty Australian Open tennis tournament.

Champion player Rafael Nadal was reported to have evaporated over 4 kilograms of sweat during his 4 hour 8 minute quarter-final win over Denis Shapovalov.

That’s the equivalent of brewing a 250 millilitre cup of tea via the energy of his own sweat-liberated-heat – every 2.2 minutes of the match.

Read more: Health Check: how to exercise safely in the heat

So how is this all related to humidity?

Sweat evaporation is dictated primarily by the wetness of the skin, and the absolute humidity of the air. Absolute humidity is a function of the amount of water vapour in the air.

This is not to be confused with relative humidity which is the term usually used in weather reports. This tells us how much water vapour the air is holding (as a percentage) relative to how much it could possibly hold, which goes up with temperature.

The bigger the difference between the wetness of the skin and the absolute humidity, the more readily sweat evaporates.

Read more: Why do I sweat so much?

In arid climates, such as central Australia, where the absolute humidity is low, sweat evaporates so rapidly you can barely feel it on your skin.

Whereas in humid conditions, such as in northern Australia and more recently much of eastern Australia, sweat evaporation is hindered.

This is why on humid days we think we’re much sweatier, when in reality we may not be sweating that much more than any other warm day. It’s just more sweat is staying on our skin (rather than evaporating), forming patches on our clothes and making us feel sticky.