Weather - Everglades National Park (U.S. National Park Service) Park visitors need to be aware of the heat index, which is a measure of how hot it feels. Other names for the heat index include the felt air temperature or apparent temperature. The heat index combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature. For example, for a temperature of 90 degrees F (32 degrees C) experienced under very high humidity, the heat index is about 105 degrees F (41 degrees C). The human body normally cools itself by perspiration, or sweating. Evaporation of that sweat removes heat from the body; however, relative humidity reduces the evaporation rate because the higher moisture content of the surrounding air inhibits evaporation. The lower heat removal rate from the body results in the sensation of feeling overheated. The effect is subjective, and its measurement is based on descriptions of how hot subjects feel for a given temperature and humidity. Visitors are advised to consider the heat index when planning activities in the park.