Chilling Reports

This weekend, Mt. Washington in New Hampshire set a record for the lowest measured wind chill. I've always been a bit bothered by the idea of wind chill, since it doesn't represent what temperature it is, but instead what temperature it feels like, which seems a bit subjective. I thought I'd take a closer look at how wind chill is calculated.

There are several different models for calculating wind chill, but the principle is the same: In calm conditions, a boundary of warmer air will build up between your body and the colder air, which will slow the heat loss, but strong winds will replace that boundary with the cold air, cooling you faster. In spite of this faster cooling, you'll never get colder than the actual temperature of the air, so if you sit on top of Mt. Washington you can rest assured you won't go all the way to -108°F, but instead stay at a balmy -47°F.

Thinking about it a bit more, I suppose my real problem with wind chill is that it's using the wrong units for what it's measuring: Wind chill is meant to convey the rate of heat loss, so that's how it should be measured. That's what the original model, called the wind chill index, measured with units of kilocalories/hour/meter^2. What's nice about this is that you can clearly see that your total energy loss will depend on how long you're outside, and how exposed you are.

If you look at the Wikipedia page above, you can see the different equations for the wind chill index and the wind chill temperature. On the surface they look similar, but the wind chill temperature has a very unusual 0.16 power, which only makes me more uncomfortable with the measure. We can look at how the two compare for a range of temperature and wind speed values:

The shapes are similar, but the wind chill index has more of a bulge, indicating that it is more sensitive to the wind speed than the wind chill temperature.

The Wikipedia article states "Many formulas exist for wind chill because, unlike temperature, wind chill has no universally agreed upon standard definition or measurement. All the formulas attempt to qualitatively predict the effect of wind on the temperature humans perceive." It seems a bit silly to tout a record based on an arbitrary perception of temperature – Ever since chemo, I've found myself more easily chilled than I used to be, so maybe I'll define my own wind chill measure: The usual wind chill temperature, minus 5 degrees. New record set!