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Under high air temperature and humidity (high heat index) conditions, perspiration is hindered due to reduced evaporation as a result of high humidity. This temperature can have potentially severe medical effects. Below is a chart based on the NWS equation that can be used to estimate temperature and the level of danger associated with varying relative humidity percentages.Īs described above, the heat index is the temperature equivalent perceived by humans as a result of air temperature, relative humidity, and wind speed. The equation used by the NWS to estimate heat index was developed by George Winterling in 1978, and is meant to be valid for temperatures of 80☏ or higher, and relative humidity of 40% or more. As such, depending on how significantly these assumptions vary from the reality of an individual, heat index estimates may not accurately reflect the perceived temperature. Like the Wind Chill Temperature Index, the heat index used by the National Weather Service (NWS) in the United States is based on many assumptions such as body mass, height, clothing, individual physical activity, blood thickness, and wind speed. This perception of heat is what the heat index seeks to measure, and while it can technically be used indoors, it is most often used in reference to the outside temperature. The lower rate of evaporation subsequently lowers the rate at which the body cools, increasing the perception of heat. The human body cools itself through perspiration, where heat is removed from the body as a result of the evaporation of sweat. A higher relative humidity affects normal body cooling by reducing the rate of evaporation of sweat. The perception of heat is subjective, and can be affected by various factors such as menopause, pregnancy, and the effects of drugs or withdrawal, as well as differences in hydration, body shape, and metabolism. This difference in perceived and actual temperature is the result of a mixture of air temperature, relative humidity, and wind speed. For example, an air temperature of 83☏ with a relative humidity of 70% would result in an estimated 88☏ perceived temperature. Heat index is often referred to as humiture, and is similar to wind chill in its attempt to measure the perceived rather than the actual temperature. Related Wind Chill Calculator | Dew Point Calculator

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