Heat Index Calculator National Weather Service

National Weather Service Heat Index Equation:

\[ HI = -42.379 + 2.04901523 \times T + 10.14333127 \times RH - 0.22475541 \times T \times RH - 0.00683783 \times T^2 - 0.05481717 \times RH^2 + 0.00122874 \times T^2 \times RH + 0.00085282 \times T \times RH^2 - 0.00000199 \times T^2 \times RH^2 \]

Heat Index (HI):

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1. What is the Heat Index?

The Heat Index (HI) is a measure that combines air temperature and relative humidity to determine the human-perceived equivalent temperature - how hot it feels. The National Weather Service (NWS) uses this index to assess heat stress on the human body.

2. How Does the Calculator Work?

The calculator uses the NWS Heat Index equation:

Where:

\( T \) — Temperature in degrees Fahrenheit
\( RH \) — Relative humidity percentage

Note: This equation is valid only when temperature is 80°F or higher and relative humidity is between 0% and 100%.

3. Importance of Heat Index

Details: The Heat Index is crucial for assessing heat-related health risks. Higher values indicate greater risk of heat disorders like heat exhaustion or heat stroke, especially with prolonged exposure or physical activity.

4. Using the Calculator

Tips: Enter temperature in °F (must be ≥80°F) and relative humidity as percentage (0-100%). The calculator will compute how hot it feels to the human body.

5. Frequently Asked Questions (FAQ)

Q1: Why does humidity make it feel hotter?
A: High humidity reduces the body's ability to cool itself through sweat evaporation, making the temperature feel hotter than it actually is.

Q2: What are dangerous Heat Index levels?
A: NWS considers 90-105°F as "extreme caution", 105-130°F as "danger", and above 130°F as "extreme danger".

Q3: Does this work for temperatures below 80°F?
A: No, the equation is only valid for temperatures of 80°F or higher. Below this, humidity has minimal effect on perceived temperature.

Q4: How is this different from "feels like" temperature?
A: Heat Index specifically measures the effect of humidity on perceived temperature, while "feels like" may also incorporate wind chill factors.

Q5: Are there limitations to this equation?
A: It assumes shade and light wind conditions. Full sunshine can increase heat index values by up to 15°F.