Heat Index Calculator

Calculate how hot it really feels based on temperature and humidity

🌡️ Heat Index Calculator

Calculate how hot it really feels based on temperature and humidity

What is the Heat Index?

The Heat Index Calculator computes the 'feels-like' temperature by combining air temperature and relative humidity. This measurement indicates how hot conditions actually feel to the human body due to reduced sweat evaporation in humid air.

When humidity is high, sweat doesn't evaporate as quickly from your skin, making it harder for your body to cool itself. This is why 85°F with 90% humidity feels much hotter than 85°F with 40% humidity.

The heat index is crucial for assessing heat stress risks, guiding safety decisions for outdoor activities, issuing public health warnings, and taking precautions against heat-related illnesses like heat cramps, heat exhaustion, or heat stroke.

How to Use This Calculator

Choose your calculation method: Use Relative Humidity if you know the humidity percentage, or Use Dew Point if you have dew point temperature data.
Enter the air temperature and select your preferred unit (°F, °C, or K).
For humidity mode: Enter the relative humidity percentage (0-100%). For dew point mode: Enter the dew point temperature and select its unit.
Click 'Calculate Heat Index' to see your results.
Review the heat index value, warning level, and safety recommendations provided.

Latest Insights & Guidelines

National Weather Service Alert Thresholds: Heat advisories are issued when the heat index is expected to reach 105-110°F for 2 or more consecutive days. Excessive heat warnings are issued for more dangerous conditions.

Heat Index Warning Levels: 80-90°F (Caution), 90-103°F (Extreme Caution), 103-124°F (Danger), 125°F+ (Extreme Danger). Each level corresponds to increasing risks of heat-related illnesses.

The Standard Formula: The NWS heat index formula was developed in 1978 by George Winterling. It applies to temperatures ≥80°F (27°C) and relative humidity ≥40%, using a complex multivariate equation with an accuracy of ±1.3°F.

Best Practices: Use heat index calculations only for shaded conditions. The formula does not account for direct sunlight, wind, or physical activity. For sports or outdoor work, consider using Wet Bulb Globe Temperature (WBGT) which accounts for solar radiation and wind.

Important Limitations: Heat index calculations are not suitable for direct sun exposure or strenuous activity. High humidity thresholds may invalidate readings at lower temperatures. Always use caution when conditions fall outside the valid calculation range.

Understanding Heat Index Calculations

The Heat Index Formula

The National Weather Service uses a complex regression equation: HI = -42.379 + 2.04901523×T + 10.14333127×R - 0.22475541×T×R - 0.00683783×T² - 0.05481717×R² + 0.00122874×T²×R + 0.00085282×T×R² - 0.00000199×T²×R². Where T is temperature in °F and R is relative humidity in percent. Additional adjustments are applied for specific temperature and humidity ranges.

When is Heat Index Valid?

The heat index calculation is most accurate for temperatures at or above 80°F (27°C) and relative humidity at or above 40%. Below these thresholds, the heat index may not accurately represent the perceived temperature. The formula assumes shaded conditions with light wind.

Limitations to Consider

Not valid in direct sunlight - add up to 15°F for full sun exposure
Does not account for wind speed or physical activity level
Less accurate for very low humidity or temperatures below 80°F
Individual heat tolerance varies based on age, health, and acclimatization

Alternative Measurements

For outdoor work or sports activities, the Wet Bulb Globe Temperature (WBGT) is often more appropriate as it accounts for solar radiation, wind speed, temperature, and humidity. WBGT provides a more comprehensive assessment of heat stress in active conditions.

Frequently Asked Questions

What is a dangerous heat index level?

Heat index values above 103°F are considered dangerous, with significant risk of heat cramps, heat exhaustion, and heat stroke. Values above 125°F represent extreme danger with heat stroke highly likely. Always seek air-conditioned shelter when heat index reaches dangerous levels.

Why does humidity make it feel hotter?

Your body cools itself through sweat evaporation. When humidity is high, sweat evaporates more slowly, reducing your body's ability to cool down. This makes the air feel hotter than the actual temperature. At 100% humidity, sweat cannot evaporate at all.

How much does direct sunlight add to heat index?

Direct sunlight can increase the heat index by up to 15°F (8°C) compared to shaded conditions. This is why the standard heat index calculation specifies shaded conditions. Always add extra caution when working or exercising in direct sun.

What's the difference between heat index and dew point?

Dew point is the temperature at which air becomes saturated and water vapor condenses. Heat index combines temperature and humidity (or dew point) to show how hot it feels. Dew point is a direct measurement, while heat index is a calculated 'feels-like' temperature.

Is heat index accurate at all temperatures?

The heat index formula is most accurate for temperatures at or above 80°F (27°C) and relative humidity at or above 40%. Outside these ranges, the calculation may be less reliable. For temperatures below 80°F, the actual temperature is often a better indicator of comfort.

Who is most at risk from high heat index?

Infants, young children, elderly individuals, people with chronic medical conditions, outdoor workers, and athletes are at highest risk. Those without access to air conditioning, people taking certain medications, and individuals who are overweight also face increased risk.

Should I use heat index or WBGT for outdoor work?

For outdoor work or sports, Wet Bulb Globe Temperature (WBGT) is more appropriate as it accounts for solar radiation, wind, temperature, and humidity. Heat index is best for general weather forecasting and assessing conditions in shaded areas with light wind.