The heat index (HI) or Humiture or humidex, also known as the apparent temperature, is the temperature feels to the human body when relative humidity (atmospheric moisture content) is combined with the air temperature. This has important effects on the human body's comfort. When the body gets too hot, it begins to sweat or perspire to cool and removes heat from the body. If the perspiration is not able to evaporate, the body cannot regulate its temperature and stays hot. Evaporation is a cooling process and when sweat is evaporated off the body, it effectively decreases the body's temperature. When the relative humidity is high, the rate of perspiration from the body decreases and human body feels warmer.
In contrast, when the relative humidity decreases the rate of perspiration increases and the body feels cooler as a result of dry conditions. There is direct relationship between the air temperature and relative humidity and the heat index, meaning as the air temperature and relative humidity increase, the heat index increases and vice versa. In order to determine the heat index, you need to know the air temperature and the relative humidity.
For example, when the temperature is 32oC (90oF) with very high humidity, the heat index can be about 41oC (106oF).