Heat Index Calculator
Calculate NOAA "feels like" apparent temperature from ambient temperature and relative humidity with heat exhaustion and heat stroke danger warnings.
Heat cramps and heat exhaustion likely. Heat stroke possible with prolonged activity.
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What Is a Heat Index Calculator?
The Heat Index Calculator calculates the apparent "feels like" temperature using the official NOAA National Weather Service Rothfusz regression equation. It combines dry-bulb air temperature with relative humidity to evaluate the human body’s ability to cool itself through sweat evaporation.
How to Use This Calculator
- Enter ambient air temperature in Fahrenheit (°F) or Celsius (°C).
- Enter relative humidity percentage (0% to 100%).
- Review the apparent heat index temperature and the associated NOAA risk category (Caution, Extreme Caution, Danger, Extreme Danger).
NOAA Rothfusz Regression Equation
HI = -42.379 + 2.049T + 10.143RH - 0.2248T\cdot RH - 0.006838T^2 - 0.05482RH^2 + 0.001229T^2 RH + 0.0008528T RH^2 - 1.99\times 10^{-6}T^2 RH^2Where T is ambient air temperature in °F and RH is relative humidity in percent. High humidity impedes sweat evaporation, drastically raising thermal body stress.
Worked Example
Scenario: Ambient temperature of 92°F with 65% relative humidity
Temperature T = 92°F, Relative Humidity RH = 65%.
Apply Rothfusz polynomial regression with humidity boundary adjustments.
Heat index output: 108.3°F (42.4°C).
Risk category: Danger (104°F to 124°F).
Tips & Key Notes
- In direct sunlight, add up to 15°F (8°C) to the calculated heat index value.
- Drink 2 to 4 cups of water per hour during heavy outdoor labor in Caution or Danger conditions.
- Take 15-minute rest breaks in shaded or air-conditioned environments when heat index exceeds 103°F.
Frequently Asked Questions
Why does high humidity make hot weather feel hotter?
The human body cools itself primarily through perspiration. When the air is saturated with moisture, sweat cannot evaporate efficiently into the air, trapping body heat inside.
What are the NOAA Heat Index danger categories?
1. Caution (80°F–90°F): Fatigue possible. 2. Extreme Caution (91°F–103°F): Heat cramps and exhaustion possible. 3. Danger (104°F–124°F): Heat cramps and exhaustion likely; heat stroke possible. 4. Extreme Danger (≥125°F): Heat stroke imminent.
Does the heat index apply in the shade or direct sun?
The official NOAA Heat Index is calculated for shady, light wind conditions. Exposure to full direct sunlight can increase heat index values by up to 15°F (8°C).
How does heat index differ from wet-bulb temperature?
Heat index measures how hot human skin feels in shaded conditions. Wet-bulb temperature (WBT) measures the lowest temperature achievable by evaporative cooling; a sustained wet-bulb temperature of 95°F (35°C) is the absolute survivability threshold for humans.
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