Wind Chill & Solar Radiation

February 2006

Over the Christmas holidays, I attempted two Iron Butt runs … a 1,046 mile ride–from Miami Beach (FL) to Elizabethtown (KY)–which I aborted after 743 miles due to imminent hypothermia … and a 1,194 mile ride–from Atlanta (GA) west to Birmingham (AL), south to Mobile (AL), east to Jacksonville (FL) and then south to Miami Beach–on which I survived the cold and succeeded.

Same bike and rider. Similar road, traffic and weather conditions. So, why did I fail in my first attempt? Because …

I did not properly calculate and fully respect the impacts and interactions of cooler temperatures, wind chill and the presence or absence of solar radiation.

Any distance riders from Alaska or Canada reading this article may be chuckling at this point, but Sunbelt cyclists considering a northbound winter run should heed what I have to say:

The human body loses heat when it’s exposed to cold air. The greater the wind speed, the faster it loses heat. We can feel or sense this heat loss, which is known as wind chill. To estimate the heat loss based on temperature and wind speeds, the National Weather Service (Weather.gov) created a Wind Chill Temperature (WCT) Index. This wind chill index was developed to describe the relative discomfort and/or danger resulting from a given combination of wind and temperature.

Before I left Miami Beach for Elizabethtown, I checked the weather forecasts along my route, and found expected daytime highs in the low 50s with nighttime lows in the 30s. I had ridden in 26 degree weather before, so I figured that the gear that kept me warm enough to ride then would suffice for what I’d be riding through now….

I figured wrong. My sunny daytime ride north through Florida and into Georgia was cool, but comfortable enough. Once I hit the outskirts of south Atlanta, though, Christmas shoppers slowed traffic to a crawl, the warming sun began to set, and the temperature started to plummet. By the time I hit the northern I-285/I-75 interchange, my bandanna-covered cheeks were stinging, my Gore-Tex gloved fingers felt like they were packed in ice, and I had to constantly flex my leather-chapped leg muscles to keep them from seizing. I willed myself a few miles further north to the Acworth exit, but there I succumbed to the Ramada Limited’s promise of a hot shower and a warm bed. I had ridden 743 miles, but my Iron Butt was now a Frozen Butt, and I could ride no more.

My mistake was assuming that the gear that kept me warm enough to ride in the 20s in daylight would also keep me rolling through nighttime temperatures in the 30s. I had not considered the interaction of wind chill and solar radiation. Yes, the wind chill index for 70 MPH at 26 degrees may be 3, but bright sunshine can warm the apparent temperature back up to 21. Run that same speed at night in 32 degrees, and what you feel is a bone-chilling 12!

What I learned from my failed first attempt, I applied to make my second run a (concededly shivering) success: I found a warmer way to rig my face and neck cover. I bought glove liners, and donned a pair of Under Armour (UnderArmour.com) ColdGear leggings under my jeans and chaps. I rode mostly north to south, which gave me the psychological advantage of knowing that no matter how cold it got, it would probably get warmer if I just kept going. And last but not least, I calculated rather than guesstimated my wind chill factors. You can do the same with this:

Until Next Time … Ride Long, Ride Free!