For those of you in the Northern Hemisphere this may sound jarring, but winter is here. Today was a particularly nasty day in Sydney – with apparent temperature dropping as low as 0.5 degrees Celsius. That’s in a city where anything below 10 degrees is often regarded as abnormal and snow is a freak event. Way to hammer home the fact it’s the first day of winter, officially.
You’ll note that I mentioned ‘apparent temperature’ – a measurement on weather reports also referred to as “feels like” or “wind chill”. The latter is only used if the apparent temperature is lower than the air temperature.1 That’s because skin surfaces exposed to cold air lose heat, but if the cold air is also accelerated by wind, they do so even faster. Hence we experience the cold more acutely.
Personally, particularly if it’s a cold, windy day, I find apparent temperature much more informative, because I don’t give a damn about the objective air temperature – I want to know whether it will feel like my face is freezing off. (It did today.)
How is wind chill determined?
While a thermometer might be all you need to tell what the air temperature is, a thermometer doesn’t have skin, or ears, or a nose. It has no feelings, either, and no way of telling what it will feel like when you step out of the house.
So, how do you tell the apparent temperature on a cold day? You calculate it using a wind chill formula that factors in the air temperature, wind speed at the height of an average person’s face, relative humidity, and the rate at which bare skin transfers heat when exposed to said wind.
More than one of those exists, because no universal standard is developed, and there are regional differences in weather, too. As the Australian Bureau of Meteorology explains, normally wind chill indices are devised for much colder temperatures than we feel over here, but a conversion chart can still be used to get an idea. Their model also “assumes an appropriately dressed adult for those conditions”.
While mathematical formulae are good for spitting out a number, the individual perception of temperature may still vary. I think today was about minus elebenty. I hate wind chill.