Readers reply: how is the wind-chill factor calculated?

·4 min read
<span>Photograph: Bill Hornstein/Getty Images</span>
Photograph: Bill Hornstein/Getty Images

On the weather forecasts, they always say: “With the wind, it’ll feel like …” How is the wind-chill factor calculated? Mick Rawlinson, Brighton

Send new questions to nq@theguardian.com.

Readers reply

My father refuses to believe in wind-chill factor because they didn’t have it in his day. salamandertome

The difference in perceived and actual temperature is the result of a complex mix of air temperature, relative humidity and wind speed, as well as differences in health, genetics, hydration, body shape, dress and metabolism. So, wind chill temperature is entirely and highly subjective, not an exact science. Weather services in different countries make many assumptions relevant to their particular region and population, and thus its estimates may differ from those provided by local weather services in other regions. HaveYouFedTheFish

Wind chill = 13.12 + 0.6215T – 11.37 (V0.16) + 0.3965T (V0.16)

T = Temperature in degrees Celsius
V = Wind velocity in kilometres per hour

Alternatively, it can be measured in brass monkeys. Today, it is four. woodworm20

Multiply the wind speed by 0.7. Then subtract that value from the air temperature. So in Bodalla, NSW, on 1 December, the first day of summer, with a wind speed of 7km and air temperature of 25C, the chill factor is 20.1. Not too chill at all. Ursula Nixon, Bodalla. NSW, Australia

There is a popular misconception that wind chill lowers the temperature. It doesn’t. The ambient temperature can never fall below the still-air thermometer reading. The wind merely serves to accelerate the rate at which exposed skin falls to that level. So a strong wind at 0C produces a net rate of body heat loss equivalent to that on a calm day of around -10C.

So long as you keep your skin covered (hat, gloves, good windproof top layer), wind chill doesn’t affect you. Because of this, I climbed a Scottish mountain 10 days ago in sideways rain and 45-50mph winds (force 8-9) in two base layers and a thin mid layer, plus a good waterproof top. Ambient temperature 5C, “feels like” -5C, but I didn’t notice. Even when resting at the summit in the lee. Rickytip

If your nuts and bolts freeze, its chilly. MysticKitty

Frankly, if it’s cold enough to freeze the rowlocks off a brass dinghy, it’s cold. javathedog

It’s the same answer every time which makes it easy.

“According to the car, it’s 3C – and with the wind-chill factor it’s fucking freezing.”

“Apparently it’s going to be -1C tomorrow, and with the wind-chill factor it’ll be fucking freezing.”

And so on. idontcareanymore

I would imagine that “wind-chill effect” is directly dependent on any individual body’s ability to re-warm the affected body area. As far as I’m aware this can only really be done by the circulation of warm blood in the affected area. So, someone with an exposed area which suffers from an arthritic condition will most likely experience wind chill much more than someone without any health compromises. It’s really about core body heat being “chilled” and the difficulties with restoring protection. Surface chilling can be protected against. The real problem is when it penetrates and becomes difficult to resolve by default. NewMe359

I remember being surprised to discover that, even indoors, air temperature is not the only thing that affects how “warm” a room is, from a human point of view. Heat leaves the body via radiation as well as through contact with the air; cold walls or ceilings can increase this loss rate, and make an appreciable difference to “warmth”, even if a thermostat holds the air temperature steady. Really sophisticated thermostats even sometimes measure the outdoor temperatures and bump the indoor air temperature setting up or down to compensate for this effect. Filterreader

How to calculate wind chill with an explanation can be found here.

Wind chill factor was designed for a limited purpose. It measures heat loss from exposed areas of the human body, like hands and face, in low temperatures and measured wind speeds. With this information, people venturing outside in exceptionally bitter weather would know an approximate time before exposed areas of the body would succumb to frostbite. BijiDog

Not being good at figures but being a resident of Canada, when I hear that the wind chill is making the temperature considerably colder outside, I remember to wear a hooded coat, a woolly scarf, and proper gloves or mittens, to say nothing of comfy boots with good traction. I also sport a beard, an added adornment that also helps to prevent frostbite. Richard Orlando, Westmount, Quebec


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