It’s not a trivial question for cyclists. Too little, too much, or the wrong apparel can make a ride miserable, or, worst of all, impossible. As a new bicyclist, I learned quickly that general guides weren’t all that helpful for me: Thermal comfort is highly individual, and I’m the only one who knows what works best in my specific case.
So I started experimenting on short rides, to ensure that I was well-prepared for long ones. See those nearly-illegible, messy cards above? Each time I came home from a ride, I noted the actual temperatures, wind speed (if relevant), what I wore, and added any other helpful notes. This has become a quick and simple part of my post-ride routine.
It didn’t take long to build up a library of useful lists. Before a ride — particularly a long one, far from home — I pull out this little notebook (it’s just the index cards clipped together) and compare the day’s forecast to my previous notes. I may slip up sometime in the future, but so far, this system has worked incredibly well, and I’ve been able to calculate what bicycling clothing I need at any given time with surprising accuracy.