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Weather forecasters often give two tempertures, the actual air temperature and a 'feel like' temperture that takes into account wind speed. Is there a rule of thumb for calcualting the 'feels like temperature'? Something like -1 °C for every 0.5m/s wind.

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Temperature and wind speed individually are more important than wind chill. You won't get frostbite at 33F, no matter how hard the wind is blowing, and you certainly need to dress differently if it's cold and the wind is blowing 40 mph. –  xpda Feb 3 '12 at 16:53
@xpda at 33°F you might not get frostbite, but wind chill could still affect how you need to dress (above 50°F the effects of wind chill are minimal). You've got to consider temperature and wind speed together because they act in synergy. –  Graham Feb 3 '12 at 17:58
Yes, but windchill alone is insufficient. –  xpda Feb 3 '12 at 23:21
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4 Answers

up vote 6 down vote accepted

Personally, I would use a wind chill chart, e.g this keyring compass includes a wind chill chart that would be easy to carry. It's still not going to be accurate, but it would provide a guide when you have nothing better.

Take a look at the Wikipedia page on wind chill, the calculations look a bit "frightening" - not something I would like to do in my head.

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Interestingly, only one of those calculations accounts for relative humidity, which always seems (to me) to be a pretty big factor in "feels like temperature". A damp cold often seems worse to me than a dry cold. –  D. Lambert Feb 3 '12 at 14:38
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The whole concept of wind chill is not very meaningful. This can be obscured by all the fancy-looking formulas, which lend it an air of precision. It's little better than junk science, especially if used uncritically by people who assume that it must be meaningful because it's a mathematical formula.

If you're completely covered and have an outer layer that blocks the wind completely, then there is very little difference between windy and calm conditions. More typically, you might have boots that block the wind completely, some wool layers on your upper body that block the wind partially, and skin completely exposed on your face. In this situation, the wind chill is almost irrelevant for your feet, is partially meaningful for your upper body, and has a full impact on your face. In this example, your ability to maintain your core body temperature would be determined by the total rate at which you were losing heat in all these different parts of your body. There is no way that these factors are going to be correctly described by a generic formula.

As Graham has pointed out in a comment, a lot of this also depends on what health risk you're talking about.

Suppose the temperature is 35 F (2 C), the wind is 40 mi/hr (65 km/hr), and you're wearing a couple of thin layers of cotton clothing and soaking wet. In this situation, you are almost certainly going to die of hypothermia unless you can find shelter, but your risk of frostbite is probably zero. The fact that you're going to die of hypothermia is almost completely determined by the wind. If you could get out of the wind, you'd probably survive.

But say the temperature is -20 F (-29 C), the wind is 20 mi/hr (30 km/hr), and you're clothed in a thick wool base layer, a wool sweater, a thick down jacket, goggles, and a balaclava -- but you're wearing lightweight goretex boots. If you keep moving, you're probably not going to get hypothermic, but you're at risk of frostbite on your toes. The wind chill is a non-issue, because the only risk is frostbitten toes, and your toes are sealed inside the boots where they're not exposed to the wind. It's true that the rate of heat convection away from the outside surface of your boots is somewhat influenced by the speed at which air flows over them, but this is a relatively minor effect here.

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This question has been answered already, but this problem interested me and I thought I'd share for future readers what I found by playing around with it.

The North American Wind Chill Index as presented by NOAA here is based on the formula (itself an approximation),

enter image description here

The chart applies to temperatures T from 40F to -45F and wind speeds 5mph to 60mph, and is said (by Wikipedia) to be accurate to within about a degree. The hard part above (apart from decimals which are hard to remember) is the V^{0.16} part. We can approximate that with a line-of-best-fit (rounded) to get

enter image description here

We can plug this into the NOAA formula to get the following linear approximation on that range of wind speeds:

enter image description here

This still doesn't help with the hard-to-remember decimals, so we can round to the nearest easy numbers to get the following rule-of-thumb:

enter image description here

It's still a formula, but it makes a whole heck of a lot more sense than the NOAA formula. What me might really like to know, though, is: how good is the approximation? I set up some tables so you can compare the three different methods.

First, the NOAA advertised method:

enter image description here

The table is set up just like the chart on the website - wind speed increases going down the left column, and temperature decreases going left to right.

Next comes the linear approximation, rounded to the nearest degree:

enter image description here

As you can see, the linear approximation is within a degree or so of the NOAA chart (I didn't see anything farther off, but I didn't check the whole table, either).

Finally, the easier-to-remember version, again, rounded:

enter image description here

The rule-of-thumb version isn't as close as the linear approximation, but looks to usually be within a few degrees. The discrepancy looks largest at the boundaries (V = 5 and V = 60), which makes sense, because the linear approximation is worst at the boundaries. Definitely the above shouldn't be used for wind speeds larger than 60mph, but in that case, a person should be inside curled up by a fire rather than figuring crude wind chills anyway.

I suppose a person could come up with all sorts of rules-of-thumb, depending on the method used to approximate V^{0.16}. I don't know the details of how NOAA came up with their published approximation, but there might be something there, too, a person could use to come up with something that might work in a pinch.

Edit Going back to the original question, we can say that for a fixed wind speed, if the temperature drops 5F then the wind chill temperature drops approx 6F. For a fixed temperature, the wind chill temp drops approx 1F for each additional 3mph of wind speed. Both of the above are neglecting a little bit (of V/200 and T/200, respectively), and are only valid in the given ranges.

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Relative humidity is doing to be a big factor here, both on the too wet and too dry sides. Staying warm is generally not something you want to try to do "by the numbers". If you're out in the cold and wind, you need to pay attention to the feedback your body is giving you about the conditions you are in. Pay attention for the signs of hypothermia, frostbite, and/or dehydration and react accordingly. Don't just check the windspeed and temperature and rely on that to determine who fine you are.

If you are planning in advance of a trip and want to know how cold it will be, don't use a rule of thumb, use a real calculation, but don't forget to add in humidity. Generally you should carry enough extra clothes to cover any extreme weather changes anyways.

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