“Undoubtedly wind chill is a very real phenomenon that can have a dramatic impact on our bodies. It is, however, a parameter which has huge variation from person to person and depends significantly on how we dress, what our environment is like and on many factors where there will always be some level of uncertainty or variation. While we have gotten used to explicit declarations of the wind chill value, reality is far more fuzzy.” – Brad Vrolijk, The Problem with Wind Chill (at “A Weather Moment” blog)

“If the weather makes headlines only when it’s horrendous out, wind chill is its PR agent.” – Daniel Engber (“Why are we still reporting on ‘Wind Chill?‘” at Slate.com)

On Monday, temperatures in the Boston area dropped sharply throughout the day, and by late afternoon, the combination of temperatures in the low 20s and a no-nonsense Northwest wind made running outdoors an extreme adventure. Around three o’clock that afternoon I left my desk to sneak in a run before it got dark. Heading down to the locker rooms to change, I briefly considered running on a treadmill in the building’s Health Center, but all of the machines were in use and I wasn’t inclined to wait. Instead, I pulled on every layer of extra clothing I had brought with me, and ventured out into the howling outdoors.

Except in those rare cases when the weather gods deliver a 20-mph tailwind on a point-to-point course (see Boston 2011), strong winds are the bane of runners. In mild conditions, strong winds can be a nuisance, but when it’s really cold, strong winds flay exposed skin and generally make a runner’s life miserable.

Of course, it’s not just runners who worry about this. Beginning about 75 years ago, scientists began looking at ways to quantify and express the combined effects of wind and cold, both the rate of actual heat loss experienced by a warm body in a cold environment, and the perception of temperature based on that rate. Hence, the “Wind Chill Factor” or, as it is likely to be reported these days, the “Feels Like” temperature.

But for a sensation that is all-too-familiar — feeling colder when it’s windy — it’s surprisingly hard to quantify this effect.

The problem with wind chill is that while it sounds scientific, it’s notoriously imprecise and based on a number of questionable assumptions, for example, assumptions about a person’s height, width of face, personal thermal conductance, direction and speed of travel relative to the wind direction, absence of solar radiation, etc. Variations in these inputs affect the outputs to a rather large extent, so that unlike actual temperature, which is easy to measure, “Feels like” temperature is very hard to measure, and inherently subjective.

There are other problems. Expressing wind chill as a temperature strongly encourages incorrect conclusions about actual effects, for example, when the air temperature is above freezing but the wind chill number is below 32 degrees F. In that case, the effect of the wind is still felt in heat loss, but there is no chance of frostbite, or pipes freezing, or any other effects associated with an actual temperature below 32.

On the other hand, when running, sometimes wind chill UNDER-estimates the misery of battling the wind and cold.

One of the first rules of cold-weather running is to begin runs heading into the wind so that there is less build-up of sweat in the early part of the run and less chance of hypothermia when you turn around and face an icy blast with damp clothes and skin. I once made the mistake of ignoring this rule on the Boston Marathon course. I parked in Hopkinton and, with temperatures in the mid 20’s and a nice tailwind, I had a lovely 9-mile run to West Natick. When I turned around to run back to my car, it was just brutal, and the hour that it took me to get back to my car was one of the coldest of my life.

It so happens that when I run from work, I’m pretty much forced to head East and downhill for the first part of my run. Consequently, on Monday afternoon, I ran my first few miles in mortal fear of the misery that awaited when I had to climb back up the hills with 20 mph gusts in my face. Luckily, it was too cold to sweat, even with the wind behind me, so I survived the return trip, although I didn’t linger outside after finishing.

In short, using wind chill factor as a guide is pretty useless compared with common sense and smart gear choices. I’ve become an expert in layering, and conscientious about covering up every bit of exposed skin with either fabric or anti-chapping ointments. I also pay attention to the absolute temperature, and wind direction. But do I consult the wind chill factor, or believe those charts that tell me what my run will feel like when the Northwest wind is scattering empty garbage cans across the road? No, I do not.