Heat Index (HI), given in degrees F, is an apparent temperature index that describes how hot the human body ‘feels’ given the air temperature and the amount of water vapor in the air. The amount of water vapor in the air affects the apparent (or perceived temperature) because the rate of cooling slows as vapor content increases. The HI is calculated using air temperature and relative humidity or dew point temperature. The apparent (or perceived) temperature generally is greater than the actual temperature when the air temperature is above 80°F and the relative humidity is greater than 50%.