You've checked the temp on your phone and it reads 13.5 degrees Celsius, but the "feels like" temperature says it's 6.7 degrees.

So, should you bring a jacket or not?

We spoke to the Bureau of Meteorology (BOM) to find out which one is the best measure if you're going to be outdoors and how they work out the "feels like" temperature.

Here's what they told us.

The "feels like" temperature, or the apparent temperature (AT), shows how the average person would feel outside in the shade, if they are dressed for the weather.

"Effectively it attempts to quantify just how much hotter or how much colder it will feel for a person outdoors," meteorologist Matthew Marshall said.

La Trobe University academic Dr Robert Steadman helped develop the AT scale, based on how heat loss or gain — through blood flow, breathing and sweating — interact with the temperature and atmosphere.

Part of how the BOM works this out is by surveying adults on their susceptibility to the temperature. The results help develop a more mathematical model of heat balance in the human body, which looks like this:

AT = Ta + 0.33E - 0.70WS – 4.00

The BOM says in summer, high humidity can make you feel warmer than it might actually be. ( ABC News: Ian Cutmore )

To translate: AT is based on the actual temperature recorded (Ta), which involves using thermometers sheltered from the sun and wind in something called a "Stevenson screen", as well as the humidity and wind.

Humidity is measured by the "E", which looks at the water vapour pressure, while "WS" measures wind speed at an elevation of 10 metres.

As for the -4.00, it's a correction term. The short answer is that it's a simple approximation of a much more complex formula, and it keeps the errors low.

We asked ABC weather presenter Nate Byrne to explain further:

"The apparent temperature calculation is an approximation. It's slightly simpler than it really should be for ease of calculation," he said. "Essentially, accuracy is lost for convenience, and after analysing Australian conditions, the -4 correction proved to be the best least-squares fit for us (the lowest amount of error from the longer, more difficult calculation).

A Stevenson screen, closed (left) and open (right), containing a resistance temperature device (thermometer) and a relative humidity probe. ( Supplied: Bureau of Meteorology )

The BOM has pointed out that the formula is not as rigorous as the meteorology it usually uses because it's based off how people feel.

It also assumes you're in the shade and you're wearing "weather-appropriate" clothing. Here's the BOM:

"If your clothing is wet, for example, wind chill can be more severe and your chance of hypothermia would be greater than indicated by the apparent temperature."

What do humidity and wind have to do with it?

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Quite a bit. Wind chill and humidity is what makes you feel like it's warmer or cooler than it is.

Wind chill:

In winter, wind can make people feel much colder than the ambient temperature. The BOM's senior meteorologist Jenny Sturrock explains in their video (see above) that it's because of wind blowing away the heat created from your body temperature:

"You have a layer of warmer air around your body that can insulate you from cooler surrounding air [but] wind strips this layer away and allows you to feel more of the cold," she said.

"[So] closer to sea level, you might be walking your dog on a 22C day, so you don't take a jumper, but the wind chill makes it feel like 15C — quite uncomfortable."

The "feels like" temperature shows how the average person would feel outside in the shade, if they were dressed for the weather. ( AAP: Joe Castro )

Humidity:

The BOM says in summer, high humidity can make you feel warmer than it might actually be.

"When we are hot, our bodies sweat to cool us down. When there is a lot of moisture in the air, the rate at which sweat can evaporate from [and cool] the skin is reduced," Ms Sturrock said.

Humidity can make you feel warmer than it might actually be. ( Flickr.com: Kullez (CC-BY-2.0) )

Mr Marshall said it's like taking a drink can with condensation outside. In dry air, the condensation will evaporate pretty quickly. But in humid conditions, the condensation takes a lot longer to evaporate. And it's the same with when you sweat.

Why does BOM's 'feels like' only take into account the shade?

There are quite a few different "feels like" models used around the world, with the AT index used by the United Kingdom, New Zealand and the United States.

But the one BOM uses doesn't take into account radiation from the sun.

Mr Marshall explained it's because the Bureau doesn't really have a measure for it.

Instead, it's estimated the "feels like" temperature can vary up to 8C if you're outdoors but not in the shade.

So if the "feels like" temperature says it's 15C at 12:00pm, it could actually feel as high as 23C in the sun.

The other form of AT includes radiation and looks something like this: AT = Ta + 0.348E − 0.70WS + 0.70Q/(ws + 10) − 4.25

In some ways it's similar to the existing formula but the point of difference is that the "Q" takes into account the net radiation absorbed per unit area of body surface (which is the impact of the sun).

Are there any other measures for 'feels like'?

Wearing multiple thin layers is key to staying warm. ( 702 ABC Sydney: Amanda Hoh )

BOM said the wet bulb globe temperature (WBGT), which was developed in the 1950s, can also be used to measure temperature and humidity in a single number.

It's also affected by wind and radiation and is often used in occupational health and safety guidelines for working in hot environments.

But the WBGT provided by the Bureau is only an approximation — it assumes only light winds and fairly sunny conditions.

Other indices such as the physiologically equivalent temperature (PET) and the predicted mean vote (PMV) can also be used.



Is there a record for difference between real and 'feels like'?

No, according to Mr Marshall.

But BOM said in the tropics, there have been examples where the air temperature is 29C but it feels like 38C.

That can also be the case in winter.

On Tuesday, in Moss Vale, NSW the temperature was 8.4C at 6:40pm but the "feels like" temperature was a cool -1.2C.