Temperature and performance
The quantity of energy (oxygen, glucids, etc.) consumed by a sportsman is as big as the effort is long.
An important part of this energy (75%) is transformed not into mechanical form but thermical that the body is obliged to dissipate to maintain the cellular activity (thermolyse).
The capacity of thermoregulation, which is more or less big according to the sportsman, is a limitation of the performance in the same way as glucid and water reserves (Ref.).
Contrary to the water which evacuates easily by conduction the produced heat, the air evacuates it with difficulty from the skin. To improve this process our body uses the evaporation cooling from perspiration.
For all endurance efforts, it is important to take care of weather conditions (temperature, wind, etc.) because they favour or not the thermic regulation.
Then, for a better efficiency, the training runs must be chosen in the best climate conditions.
Running long distances in summer, in the full heat of day, is obviously not the best thing to do !