The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This is important because the human body cools itself through perspiration, but when humidity is high, sweat doesn't evaporate as efficiently, making it feel hotter than the actual air temperature.

The heat index is calculated using a complex formula that takes into account air temperature and relative humidity. The formula used by the National Weather Service is based on multiple regression analysis of various temperature and humidity values. Our calculator uses the same scientifically validated algorithm to provide accurate results.

The heat index is a crucial indicator of heat-related health risks. As the index rises, so do health dangers. It helps in understanding the risk of heat exhaustion and heat stroke. Authorities often issue heat advisories based on the heat index to protect public health during hot weather.

Heat index values are categorized into four risk levels: Caution (80-90°F), Extreme Caution (90-103°F), Danger (103-124°F), and Extreme Danger (125°F+). When the heat index reaches 103°F or higher, heat-related illnesses become likely with prolonged exposure, and conditions become dangerous at 125°F or higher.