As the temperature outside begins to rise, the difference between the normal skin temperature (90° F) and the ambient temperature narrows. This difference is known as the delta T. As the ambient temperature rises above 80° F, the delta T is not great enough to allow the body's internal heat to flow away from the body by convection.

Instead, the body reacts by cooling itself through a process known as evaporative cooling. When water fluid is exposed to warm, dry air, it will evaporate into water vapor. This change of state is called a phase change and it produces a tremendous cooling effect. The body creates this phase change by secreting perspiration from our sweat glands to the surface of the skin. When the air surrounding the skin is warm and dry, this is an extremely efficient process. But as the humidity rises, perspiration can no longer evaporate into water vapor, as the air is already saturated. This is a dangerous condition, since the body has no other natural mechanism.