What is heat index and how is it calculated?

Heat index, or 'apparent temperature,' is what the temperature feels like to the human body when humidity is factored in. It's calculated using the NOAA/NWS Rothfusz regression equation, which combines air temperature and relative humidity. At 90°F with 65% humidity, the heat index is about 103°F - significantly hotter than the actual air temperature.

Why does humidity make it feel hotter?

Your body cools itself through sweat evaporation. High humidity slows this evaporation, making it harder for your body to cool down. At 95°F with 90% humidity, the heat index reaches 127°F because sweat barely evaporates. Conversely, dry heat (low humidity) feels cooler because sweat evaporates efficiently.

What are the NOAA heat danger categories?

NOAA defines five heat danger levels by heat index: Normal ( 124°F) - heatstroke highly likely.

What is dew point and why does it matter?

Dew point is the temperature at which air becomes saturated and water vapor condenses into dew. A dew point above 65°F feels muggy, and above 70°F is oppressive. Unlike relative humidity, dew point is an absolute measure of moisture - a 60°F dew point feels equally humid whether it's 70°F or 100°F outside.

What is wet bulb temperature?

Wet bulb temperature is the lowest temperature achievable through evaporative cooling alone (like a wet cloth in front of a fan). It's critical for HVAC design, outdoor worker safety, and climate research. A wet bulb temperature of 95°F (35°C) is the human survivability limit - the body cannot cool itself regardless of shade or hydration.

When should I worry about heat-related illness?

Take precautions when heat index exceeds 90°F. Symptoms to watch: Caution zone - fatigue, headache; Extreme Caution - muscle cramps, heavy sweating; Danger - nausea, dizziness, rapid pulse; Extreme Danger - confusion, hot/dry skin, unconsciousness. Call 911 immediately for heatstroke symptoms.

Heat Index Calculator

NOAA/NWSHeat SafetyWeather

NOAA/NWSHeat Safety

Enter temperature and humidity to calculate the heat index and related metrics

Frequently Asked Questions

Common questions about this calculator

Heat Index Calculator

Temperature Input

Humidity

Wind Chill (Optional)

Output Preferences

Frequently Asked Questions

Heat Index Calculator

Temperature Input

Humidity

Wind Chill (Optional)

Output Preferences

Frequently Asked Questions

01What is heat index and how is it calculated?

02Why does humidity make it feel hotter?

03What are the NOAA heat danger categories?

04What is dew point and why does it matter?

05What is wet bulb temperature?

06When should I worry about heat-related illness?