Heat Index Calculator

Heat index, also called "apparent temperature" or "feels like" temperature, combines air temperature and relative humidity to determine what the temperature actually feels like to the human body. High humidity reduces the body's ability to cool through sweat evaporation.

Humidity affects how efficiently your body can cool itself through sweat evaporation. At 100% humidity, sweat cannot evaporate, making it feel much hotter. At 90°F with 30% humidity, it feels like 90°F. At 90°F with 80% humidity, it feels like 113°F.

Heat index above 103°F is considered "Danger" level where heat exhaustion is likely. Above 125°F is "Extreme Danger" where heat stroke is highly likely. Even "Extreme Caution" levels (91-103°F) can be dangerous for at-risk groups.

High-risk groups include: elderly (65+), infants and children, people with chronic illnesses, those on certain medications, outdoor workers, athletes, and people without air conditioning. These groups should take extra precautions when heat index rises.

Heat exhaustion: heavy sweating, cold/pale/clammy skin, fast/weak pulse, nausea, tiredness. Heat stroke (emergency): high body temp (103°F+), hot/red/dry skin, fast/strong pulse, confusion, unconsciousness. Call 911 for heat stroke.

Heat index uses the Rothfusz regression equation developed by the National Weather Service. It takes air temperature and relative humidity as inputs. The formula includes multiple polynomial terms and adjustment factors for extreme humidity conditions.