🌡️ Heat Index Calculator

The Heat Index, also known as apparent temperature, is a measure of how hot it feels when relative humidity is factored in with the actual air temperature. It represents the temperature equivalent perceived by humans due to the combined effects of air temperature and humidity.

This calculator uses the National Weather Service (NWS) formula developed by George Winterling in 1978. It's designed for temperatures of 80°F (26.7°C) or higher and relative humidity of 40% or more. Results are estimates and individual perception may vary.

High humidity reduces the evaporation rate of sweat, which is the body's natural cooling mechanism. When sweat doesn't evaporate efficiently, the body retains more heat, making the temperature feel hotter than it actually is.

The risk levels indicate potential health effects: Safe (minimal risk), Caution (fatigue possible with prolonged exposure), Extreme Caution (heat cramps and heat exhaustion possible), Danger (heat cramps and heat exhaustion likely, heat stroke possible), and Extreme Danger (heat stroke highly likely).

While the Heat Index can technically be used indoors, it's primarily designed for outdoor conditions. Indoor environments may have different factors like air conditioning, ventilation, and reduced sun exposure that affect comfort levels.

Direct sunlight can increase the Heat Index by up to 14°F (8°C). The standard Heat Index calculation assumes shady conditions with light wind. Full sun exposure significantly increases heat stress.

Children, elderly individuals, people with chronic health conditions (diabetes, heart disease), those taking certain medications, and individuals who are overweight or dehydrated are at higher risk of heat-related illness.