Are there any reliable and/or easy to use “rule of thumb” guides for windchill calculations?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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),

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

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

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:

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:

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:

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:

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.