Large parts of Australia’s east have woken to their coldest morning of the year in the past two days, as icy weather continues to blast the country ahead of the winter solstice.
And if the cold snap weather didn’t have you reaching for the winter woollies already, the “feels-like” temperature has been even more chilly.
According to observations from the Bureau of Meteorology (BOM), Melbourne, Sydney, Canberra and Hobart experienced a “feels-like” temperature between 3-6 degrees Celsius cooler than the actual minimum temperature on Tuesday, and similar again on Wednesday.
Tuesday’s ‘feels like’ vs actual temperature at the coldest point of day
- Melbourne: Actual temperature 3.7C. Feels like temperature 0.4C
- Sydney: Actual temperature 8.5C. Feels like temperature 2.7C
- Hobart: Actual temperature 5.2C. Feels like temperature 1.4C
- Canberra: Actual temperature -3.6C. Feels like temperature -6.1C
Meanwhile through alpine and regional areas, some towns — which were already unusually cold — plunged to well below freezing.
BOM forecaster Belinda House said this included Toowoomba’s Wellcamp Airport site, Oakey and Roma in Queensland, where the actual temperature was minus 3C to minus 4C on Tuesday morning, but the “feels-like” temperature was minus 7C.
In New South Wales’ Tablelands, Glen Innes had an actual temperature of minus 4C but a “feels-like” temperature of minus 8C.
And in the alpine area, Thredbo and Perisher in the Snowy Mountains both had observations of minus 9C, but a “feels-like” temperature of minus 12C.
Mount Hotham in Victoria, on the other hand, had a comparatively milder actual minimum of minus 6C on Tuesday, but a harsher “feels-like” temperature of minus 13C.
So why does it feel so much colder than it actually is?
All about the wind chill factor
The main factor that makes a day feel colder than the actual air temperature is the wind.
“So the reason that it was actually colder in the Snowy Mountains [on Tuesday], but felt colder on the Victorian side, was that there was a moderate breeze at Mount Hotham, whereas the other two had only a light breeze,” Ms House said.
This is because it speeds up heat loss through a process called convection, by whisking away the warm layer of air that sits just above the skin, according to Ollie Jay, a professor of heat and health at the University of Sydney.
The stronger the winds are, the faster this process occurs, which is why people fan themselves to cool down on a hot day.
“When the temperature of the air is so much lower than that of the outside of the body the temperature difference between you and the air is quite high, which means you shed quite a lot of heat naturally through this process called convection,” he said.
“The multiplier of that is effectively wind.
“If you think about the way in which heat transfers away from the body, you’ve got a micro-environment that sits just above the skin, like a pocket of air that is warmer,” he said.
“When you’ve got a higher airflow, that’s basically replacing that pocket of air at a faster rate.”
How the BOM calculates the ‘feels-like’ temperature
To try and describe this human response to temperature, the BOM uses a measure called the apparent temperature — also known as the “feels-like” temperature.
This is different to what the actual air temperature is, which is what the BOM is referencing in its forecasts.
To figure out the “feels-like” temperature, the BOM uses a mathematical model.
But there are essentially two key factors that go into the calculation.
One of them is the wind speed — which can cause significant chilling, as Dr Jay described.
The other is humidity, which — when it’s high — can make temperatures feel hotter than they are by reducing the amount of sweat evaporating off the skin.
Clothing, gender and direct sunlight
Of course, there are a lot of variables about how cold or warm it feels which are too specific to be factored into the bureau’s equations.
For instance, women will often feel colder than men, according to Dr Jay.
“Generally if…you’re more slim, particularly your limbs, the amount of heat that you lose relative to your body mass is greater,” Dr Jay said.
“So that’s why, typically, females will cool down faster than men in the same environment, because men are generally heavier, and they have a lower surface area to mass ratio.”
There are also factors like age, clothing and location that can make a difference.
The bureau’s calculation is based on the Steadman human model, which assumes you’re an adult, walking outdoors, in the shade.
This is because standing in the direct sun can increase the “feels-like” temperature by up to 8C.
It also assumes you’re dressed appropriately for the weather.
So, if you’re standing outside in wet swimwear on a 10C day thinking it feels colder, that’s on you.
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