Heat Acclimation Training

Training for a spring race requires some extra caution. In many situations, you will have been training through the winter, so you will have no adaptation to the heat. A spring race may be cool, but there is also a risk of conditions that are warm enough (above 50f) to impair performance. Heat adaptation training can prepare you for these warmer conditions. This type of training is also valuable if you are traveling to a warmer climate for a race, or if you are training in the cool part of the day for a race in the warmer times.

http://fellrnr.com/wiki/Heat_Acclimation_Training