We were spared excessive heat during the past couple of summers, but 1999 seems to be making up for lost time, at least in New England. Whether it's El Nina, global warning, or just plain bad luck, it's been a tough one so far, and summer just began a few weeks ago. In actuality, it's a combination of the two that makes it so uncomfortable. So it's hot. Yes, runners are persistent, even in the face of unreasonable, if not unhealthy environmental conditions.
Heat is a primary byproduct of movement.
By activating muscle groups that allow for forward motion, be it running or walking, heat is created within the cells through an increase in the metabolic rate. It's that simple. At certain temperatures, this process is easy. Cool air quickly whisks away heat that is brought to the skin's surface. At a certain temperature, the air becomes too warm to whisk away all or any of the excess heat.