The athlete measures the environment that they train in periodically to understand what the body's acclimated to. Any environmental factor that affects the body's performance is measured.
The athlete then measures the environment that they are competing in. These environments can vary from each other drastically.
Depending on what the athlete's acclimated to and what the race environment is, pace suggests an adjustment in their pace/heart rate.
This takes the guess work out of preparing for more harsh environments or taking advantage of more favourable conditions.