Sydney has been sweating through oppressive mugginess for days after hitting Singapore-high levels of humidity earlier this month.
Feeling hot and feeling humid are different beasts. The latter, defined by sticky sweat and shortness of breath, is unbearable. But why?
Here is everything you need to know.
What is humidity?
Very simply, humidity is water that has evaporated into the air.
How humid it is can be observed by what is known as the “dew point” temperature – that is the temperature that is cold enough to make water condense, according to Prof Steven Sherwood from the University of NSW Climate Change Research Centre.
The more moisture in the atmosphere – or the more humid it is – the higher that dew point will be.
Alongside the dew point, relative humidity tells you how close the air is to saturation – that is, when the air holds as much water vapour as it can for a given temperature.
Warmer air holds more moisture than cooler air – so the relative humidity falls if the temperature rises but the amount of moisture in the air stays the same. The Bureau of Meteorology says that “because of its direct relationship to fluctuating temperature”, relative humidity “doesn’t provide suitable guidance on how much moisture there is at a specific location”.
Keeping that in mind, relative humidity has been falling in Australia because the land is warming faster than the ocean, Sherwood says.
But that doesn’t mean we are seeing less vapour in the air. In fact, we are seeing more. Sea surface temperatures in the Coral and Tasman seas are up to 3C above average and winds over warming ocean waters bring higher humidity to our shores.
So even though relative humidity is reducing, there is still more moisture and we feel like it is more humid.
All this can be confusing, but the bureau’s “feels like” – or “apparent” temperature can help – which takes into account wind and humidity and is listed in the “observations” section for the relevant location in Australia.
It’s for this reason that “meteorologists prefer to use dew point when analysing atmospheric moisture and inferring what the conditions may feel like”, according to BoM. This is represented by the “feels like” temperature.
Why is it so uncomfortable?
Humidity makes it harder for you to stay cool.
Humans lose heat through heat transfer pathways that are driven by temperature differences between the skin and the surrounding environment, according to Prof Ollie Jay, the director of the University of Sydney Heat and Health Research Incubator.
When it gets warm, those temperature differences become small – and the only other way we can lose heat is through producing sweat.
But it isn’t sweat that cools us down, it is when sweat evaporates. Humidity in the air determines the driving force for that evaporation, Jay says.
“If you have high humidity, you have to lose all this heat by evaporation of sweat, but not enough sweat can evaporate from the skin’s surface.”
Your skin’s temperature will increase as a result – and that is what we perceive as being hot.
“If you can’t evaporate that sweat to keep cool, then it induces discomfort,” Jay says.
When it is humid, any activity can stimulate the feeling of being hot. “You will suddenly start sweating a lot and feeling very hot because high humidity is preventing your body’s air conditioning system from working,” Sherwood explains.
“The hotter it gets and the harder you are working, the more important humidity becomes to how you feel, compared to temperature.”
A warning sign of the climate crisis?
Rising humidity is a threat from climate change that “hasn’t been appreciated by the public”, Sherwood says.
“There’s no question that ocean temperatures are breaking records all over the world and they have been pretty steadily for decades … This is probably the clearest sign of climate change,” he says.
As a result, the chances of getting really muggy weather in Australia have increased substantially, compared with what the chances would have been before, Sherwood says.
How this will affect our health is “the thing we should be most worried about”, according to Jay.
“The climate is warming, is going to have more absolute humidity, more water vapour and [it will be] increasingly more difficult to keep cool physiologically,” he says.
“That induces a strain on your body. How much work your heart has to do, how dehydrated you become, how hot your core temperature gets – all of these different things that are a result of this stress increase our risk of a negative health event.”
