How to Become a Human Windchill Calculator

Dedicated to my inquisitive godson Daniel

First, the oldstyle pre-2001 calculation:

1. Find out the temperature T in degrees Celsius and the wind speed W in kilometres per hour. If you're in Toronto, you can call Environment Canada at 416-661-0123 to hear a recorded message with current conditions.
2. Subtract T from 33C to get T_S, the difference between the air temperature and your skin temperature.
3. If W=10, add 10% to T_S; if W=20, add 30% to T_S; if W=30, add 40% to T_S; if W=40, add 48% to T_S; if W=50, add 52% to T_S; if W=60, add 56% to T_S; if W>=70, add 60% to T_S.
4. Subtract T_S from 33C to get the windchill.

Example: If the temperature is -2C and the windspeed is 30 km/h, then 33-(-2) = 35, 35*40 = 14, 35+14 = 49, 33-49 = -16, so the windchill equivalent temperature is -16C.

In early 2001, Canada and the United States harmonized their wind chill calculation methods based on new research. The new values tend to be a little warmer than the old ones, but more accurately reflect how cold it really feels.

1. Find out the temperature T in degrees Celsius and the wind speed W in kilometres per hour.
2. You can use the official formula to compute the windchill equivalent (13.12 + 0.6215 T - 11.37 W^0.16 + 0.3965 T W^0.16) or you can work it out in your head as follows and get an answer that is accurate to the nearest degree.
3. First work out what I call the 'wind factor' F. At 5 km/h, it's 52%. At 10 km/h, it rises 5% to 57%. At 15 km/h, it rises 4% to 61%. At 20 km/h, it rises 3% to 64%. At 25 km/h, it rises 2% to 66%. Beyond that, it's cold, so give yourself three bonus percentage points plus another point for every 5 km/h over 25. That makes the wind factor 70% at 30 km/h, 71% at 35 km/h, 72% at 40 km/h and so on.
4. Take the number of degrees above -21 and multiply by 5/8 to get some warmth. If it's positive, be thankful. Take the number of degrees below 28 and multiply by the wind factor to get the cold. Subtract the cold from the warmth and you get the windchill equivalent.
5. Example: As I write this, my thermometer reads -18 and the wind is blowing at 20 km/h, giving a wind factor of 64%. The warmth is 5/8 of three degrees, let's say two degrees. The cold is 64% of 46 degrees, let's say thirty degrees. That gives a windchill equivalent of -28. The official formula gives -28.02: close enough!
6. If the windchill is -25, you can get frostbitten after 'prolonged exposure'. If it's -35, it takes ten minutes. If it's -60, it only takes two minutes.