So, it's really hot out. I was in cooler climates last week, and I noticed my power numbers were significantly higher. This is in accordance with a study done on the Australian national cycling team several years ago, which found that when the temperature rises from 70 degrees to 90 degrees, cyclists' power dropped about 6.5%. (source)
What can you do to improve your performance in the heat?
Research from a study in 2002 had suggested that heat acclimation has little value (source). Nine trained cyclists trained in a self-selected way for seven days at 98.6 degrees, and saw no benefit from their seven days of heat training. Researchers concluded, "moderately intense training in the heat produced only modest variable heat acclimation and only the possibility of a worthwhile enhancement of performance. Highly trained individuals probably need a more marked stimulus to achieve substantial heat adaptation and effects on performance."
Results from that study don't seem to square with findings from a 2007 study conducted at the University of Oregon, done on Cat 1 and 2 cyclists. Researchers found that 9 days of training in the heat not only improved performance (8%)in warm weather, but also in cool weather (6%). The differences between the protocals are slight: the Oregon study subjected cyclists to more rigorous heat training, and two days more of it (9 days, total).
The results led one researcher to suggest the following: "You still have to train fast (in normal weather). But if you can then ad heat acclimation on top of that, you will get a boost. I've heard rumors of pro teams already putting this to use. Train cool, taper hot. For trained cyclists, this just could mean going for an easy spin on hot days."
Anectdotally, this seems true for me. Last year I spent a week in upstate New York, training every day in moderate temperatures. I hit new power highs, and felt terrific. When I returned, the heat in DC felt awful, but after a week or two, I seemed to be in my best form of the season. Although I couldn't match what I'd done in the cool temperatures of New York, I did ride better than I'd been riding before I'd gone to New York.
This year I spent a week in Wisconsin and hit new power highs. I'm hoping my body will acclimate to the heat in time for some target races in early August. I suppose we'll see how that goes.
This suggests that maybe what we need to maximize performance is not merely training in the heat, but periodized heat training, where we incorporate temperature into our program the same way we now plan for different levels and types of intensity.
One more thing--many cyclists prepare for the tour by spending several preceding weeks at altitude. The theory is that the thin air thickens red blood cells. Another explanation is that the cooler temperatures of the Alps also allows cyclists to, when they return, taper in the heat for a grand tour.
If this all seems kind of ambiguous and complex, that's because it is. The takeway, if we are to simplify, and maybe oversimplify it, is that we should vary the temperatures at which we train.