NWS Heat Index Calculator

Heat Index Formula (NWS/NOAA):

\[ HI = c_1 + c_2T + c_3R + c_4TR + c_5T^2 + c_6R^2 + c_7T^2R + c_8TR^2 + c_9T^2R^2 \]

Heat Index (°F):

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1. What is the NWS Heat Index Calculator?

Definition: This calculator computes the heat index (apparent temperature) using the National Weather Service (NWS) formula based on temperature and humidity.

Purpose: It helps assess how hot it feels when relative humidity is factored with actual air temperature, important for heat safety.

2. How Does the Calculator Work?

The calculator uses the Rothfusz regression equation developed by NWS:

\[ HI = c_1 + c_2T + c_3R + c_4TR + c_5T^2 + c_6R^2 + c_7T^2R + c_8TR^2 + c_9T^2R^2 \]

Where:

\( HI \) — Heat Index (apparent temperature in °F)
\( T \) — Air Temperature (°F)
\( R \) — Relative Humidity (%)
\( c_1 \) to \( c_9 \) — Regression coefficients

Note: The formula is only valid when temperature is ≥ 80°F and relative humidity ≥ 40%.

3. Importance of Heat Index

Details: The heat index indicates human-perceived equivalent temperature, helping prevent heat-related illnesses during hot weather.

4. Using the Calculator

Tips: Enter temperature in °F and relative humidity in %. Both values are required for calculation.

5. Frequently Asked Questions (FAQ)

Q1: Why does the calculator say "N/A"?
A: The NWS formula only works when temperature is ≥ 80°F and relative humidity ≥ 40%.

Q2: What are the heat index danger levels?
A: Caution (80-90°F), Extreme Caution (90-103°F), Danger (103-124°F), Extreme Danger (125°F+).

Q3: Why is humidity important for heat perception?
A: High humidity reduces sweat evaporation, impairing the body's cooling mechanism.

Q4: Does this work for Celsius temperatures?
A: No, the NWS formula is specifically calibrated for Fahrenheit inputs.

Q5: How accurate is the heat index calculation?
A: Very accurate for shaded, light wind conditions. Full sun can increase perceived temperature by 15°F.