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What is apparent temperature and how do they measure what it really 'feels like' outside?

Penny Travers, Wednesday August 9, 2017 - 07:51 EST

Did you check the temperature on your phone this morning before heading outside, only to be surprised that it felt much cooler than you had expected?

Most Canberrans can attest that the weather has been bitterly cold over the past week, much colder than the official temperatures recorded for the region.

That is because the Bureau of Meteorology's (BOM) instruments measure ambient temperature — how warm the air is in the shade, sheltered from wind.

To know what temperature it "feels like" outside, you need to consider the apparent temperature.

"It's a calculation that takes into account the actual air temperature and also other factors like humidity and wind speed," BOM meteorologist Andrea Peace said.

"At this time of the year, in southern parts of Australia, the wind is a real factor and so we're often feeling like it's a lot colder than the air temperature actually is."

How is the apparent temperature calculated?

To determine the apparent temperature the BOM uses a mathematical model of an average adult wearing shorts and a t-shirt walking outdoors in the shade.

"Humans all have this thin layer of warm air encompassing their body and when the wind blows, and particularly when it's quite cool, it basically blows away that layer of air so it leaves us feeling very exposed," Ms Peace said.

At higher temperatures, wind chill is less significant.

In the tropics, humidity plays a greater role in how warm you feel.

"When we're hot we evaporate moisture away from our skin," Ms Peace said.

"But when it's really humid the moisture acts to keep the sweat close to our body so we can't cool down in the same way.

"That's why we can often feel a lot warmer than what the air temperature is."

Useful to know when heading outdoors

So if you are heading outdoors, it can be very useful to know the apparent temperature.

You may be going out to walk the dog and the thermometer reads 22 degrees Celsius, so you head out without your jumper, only to discover the wind makes it feel 15C.

Or you might be preparing for an alpine hike in extreme conditions; the forecast may be -1C but due to high winds you should be planning for it to be a much colder -12C.

And while what you are wearing and your body shape and size will play a big part in how the temperature feels to you, the apparent reading can give you a pretty good idea of what to expect outside.

"Traditionally we would check the temperature, but if you're doing an activity outdoors where you're going to be in the elements, checking the apparent temperature or the feels-like temperature is a really good way to prepare," Ms Peace said.


© ABC 2017

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