Calculate how hot it actually feels outside using temperature and humidity.
Heat Index (also called the “feels like” temperature) combines air temperature and relative humidity to describe how hot it actually feels to the human body. This calculator uses the U.S. National Weather Service Rothfusz regression.
T is temperature in °F and R is relative humidity in %. This regression is most accurate for temperatures above 80°F and humidity above 40%.
At 95°F with 60% humidity, the heat index works out to roughly 133°F — a significant jump from the actual air temperature, illustrating why humidity makes heat feel far more dangerous.
It's the "feels like" temperature that combines actual air temperature with relative humidity, reflecting how hot conditions feel to the human body since humidity affects the body's ability to cool via sweat evaporation.
High humidity slows sweat evaporation, which is your body's main cooling mechanism, so the same temperature feels hotter and more dangerous when humidity is high.
High heat index values significantly increase the risk of heat exhaustion and heat stroke, which is why weather services issue heat advisories based on this combined measure, not temperature alone.