Click here for the National Weather Service's Heat Index Chart . Criteria for heat waves vary by location. A period of hot, humid weather in Chicago, Detroit, or New York could be considered an average summer day in Atlanta, New Orleans, or Houston. But in northern climes, sporadic hot weather is less common and is far more taxing on the population than prolonged heat and humidity in areas where people are accustomed to it. Excessive heat can kill by taxing the human body beyond its abilities to cope with extremes.
Blood vessels dilate so the blood can circulate more quickly, closer to the skin's surface.
The excess heat is then vented from the skin into the air, but only if the air temperature is lower than the body temperature. The hypothalamus also calls millions of tiny sweat glands into action. Excessive heat breaks down the body's natural cooling system. When sweat is evaporated into the air, it provides additional cooling. But too much humidity in the atmosphere prevents sweat from evaporating.