Feels like… weather
The weather forecast says it’s 15⁰C, so you head for the front door wearing jeans, a long sleeve shirt and a jumper. But as soon as you step outside, you’re hit by an icy blast and realise you’re wildly underdressed.
Maybe the weather report’s wrong. You check the forecast again and notice an extra snippet of information that reads, “Feels like 8⁰C”. So, what does that mean?
The thermometer that records air temperature for the Bureau of Meteorology (BOM) sits in the shaded, rain-free environment of a louvred wooden box known as a Stevenson screen.
The box is named after its inventor, civil engineer Thomas Stevenson; the father of famous 18th century writer Robert Louis Stevenson, who penned Treasure Island and the Strange Case of Dr Jekyll and Mr Hyde.
Stevenson screens sit atop a pole above a natural surface like grass, so the thermometer isn’t affected by heat radiating from a hard surface like cement.
They’re positioned well away from tall buildings, trees or other structures that could radiate or absorb heat.
In the southern hemisphere the screen’s door faces south, so the thermometer isn’t hit by direct sunlight even when the door’s open.
The box is painted white to reflect rather than absorb solar heat, and the louvres allow air circulation to minimize heat build-up.
Historically, meteorologists have used mercury-in-glass thermometers and checked the temperature twice a day. Many modern weather stations now use electronic sensors that can send instant temperature readouts to the BOM throughout the day.
So, from the comfort of its cosy box, the thermometer displays the ambient air temperature as degrees Celsius, named after Swedish astronomer Anders Celsius, who developed the temperature scale in the 18th century.
However, when it comes to real-world conditions, we need to consider what’s happening outside the box.
When BOM posts a ‘feels like’ temperature – also known as the apparent temperature (AT) – it uses a formula that takes into account ambient temperature, as well as wind speed and relative humidity.
Cool wind blowing over skin can rapidly dissipate body heat, making us feel colder than the ambient air temperature would indicate. By observing BOM’s ‘feels like’ forecast, you’re less likely to leave the house a jacket shy of a comfortable day out.
In high humidity, we perspire to the point that the cooling effect of perspiration evaporating from our skin is reduced, which makes us feel hotter than the forecast suggests.
Under these conditions, the ‘feels like’ figure may provide an ideal excuse to avoid competing in the City to Bay on a muggy day.
While the ambient temperature is a guide to what awaits us outside, the ‘feels like’ figure helps us more precisely plan a day at the beach, a hit of tennis or a picnic in the park.