Heat Index Converter

What is the Heat Index?

The heat index, also called the apparent temperature or “feels like” temperature, combines air temperature and relative humidity to determine how hot it actually feels to the human body. When your body sweats, the evaporation of moisture cools you down. High humidity slows this evaporation process, making you feel significantly hotter than the actual air temperature suggests.

The Rothfusz Regression Formula:

The National Weather Service in the United States uses the Rothfusz regression equation to calculate the heat index. This formula was developed in 1990 and remains the standard method used by meteorologists worldwide:

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²

Where T is the air temperature in degrees Fahrenheit and R is the relative humidity expressed as a percentage.

Celsius to Fahrenheit conversion for heat index values:

• °F = (°C × 9/5) + 32
• °C = (°F - 32) × 5/9

Heat Index Danger Levels (NWS Categories):

Practical Examples:

• At 90°F (32°C) with 40% humidity, the heat index is about 91°F (33°C), barely above the actual air temperature.
• At 90°F (32°C) with 90% humidity, the heat index jumps to approximately 122°F (50°C), which is dangerously hot.
• At 100°F (38°C) with 50% humidity, the heat index reaches about 118°F (48°C).
• At 85°F (29°C) with 80% humidity, the heat index is about 97°F (36°C).

What happens below 80°F:

The Rothfusz regression is fitted to hot conditions and returns nonsense in mild weather, giving 77°F for a pleasant 70°F day at 50% humidity. The National Weather Service handles this by computing a simpler average first and only switching to Rothfusz when that average reaches 80°F. The calculator above does the same, which is why a 79°F day at 95% humidity still comes back at 83°F rather than a flat 79. Damp air at the top end of the mild range does add a few degrees, and a calculator that just echoes the thermometer back at you is hiding that.

Important Tips:

• The Rothfusz formula is only accurate for temperatures above 80°F (27°C) and humidity above 40%.
• Direct sunlight can add up to 15°F (8°C) to the calculated heat index value.
• Athletes, outdoor workers, elderly individuals, and children should pay close attention when the heat index exceeds 90°F (32°C).
• Always stay hydrated and seek shade or air conditioning during high heat index conditions.