Heat Index Calculator
Calculate the heat index using the NOAA/NWS formula. Includes OSHA danger tier warnings for heat safety during outdoor work and sports.
Heat Index
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Safety Recommendations
Heat cramps and heat exhaustion possible. | Limit strenuous outdoor activities to early morning or evening. | Drink water frequently — don't wait until you're thirsty. | Wear lightweight, light-colored, loose-fitting clothing.
{"heatIndex":99.6777179000001,"temperature":90,"humidity":60,"dangerTier":"Extreme Caution","dangerColor":"#f97316","recommendations":["Heat cramps and heat exhaustion possible.","Limit strenuous outdoor activities to early morning or evening.","Drink water frequently — don't wait until you're thirsty.","Wear lightweight, light-colored, loose-fitting clothing."]}
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Heat Index (Feels Like)
99.7°F
Extreme Caution
NOAA Danger Tiers
Temperature
90°F
Humidity
60%
Danger Tier
Extreme Caution
Heat Index
99.7°F
Safety Recommendations (Extreme Caution)
- Heat cramps and heat exhaustion possible.
- Limit strenuous outdoor activities to early morning or evening.
- Drink water frequently — don't wait until you're thirsty.
- Wear lightweight, light-colored, loose-fitting clothing.
General Heat Safety
- Drink plenty of water before, during, and after outdoor activity — don't wait until you're thirsty
- Wear lightweight, light-colored, loose-fitting clothing and a wide-brimmed hat
- Schedule outdoor activities for cooler parts of the day (early morning or evening)
- Never leave children or pets in parked cars — temperatures can rise 20°F in 10 minutes
- Check on elderly neighbors and those without air conditioning during heat waves
The Formula
The National Oceanic and Atmospheric Administration (NOAA) Heat Index formula, also known as the "apparent temperature," combines air temperature and relative humidity to determine how hot it actually feels to the human body. High humidity reduces the evaporation of sweat, impairing the body's ability to cool itself. The formula was developed by R.G. Steadman in 1979 and adopted by NOAA for public heat safety warnings.
Variable Definitions
Air Temperature
The actual air temperature in degrees Fahrenheit.
Relative Humidity
The relative humidity percentage (0-100).
Heat Index
The apparent temperature or "feels like" temperature in °F.
How to Use This Calculator
- 1
Enter the current air temperature in degrees Fahrenheit.
- 2
Enter the relative humidity percentage (0-100%).
- 3
The calculator applies the NOAA heat index formula to compute the feels-like temperature.
- 4
Review the danger tier and safety recommendations for your heat index value.
The heat index combines temperature and humidity to measure how hot it actually feels
Understanding the Concept
The heat index (also called the apparent temperature) is what the temperature feels like to the human body when relative humidity is combined with the air temperature. When humidity is high, the evaporation of sweat from the skin slows down, making it harder for the body to cool itself. This can lead to heat-related illnesses ranging from mild heat cramps to life-threatening heatstroke. The NOAA heat index was developed for shady, light wind conditions — direct sunlight can increase the heat index by up to 15°F. The Occupational Safety and Health Administration (OSHA) uses the heat index to issue workplace safety guidelines, and the National Weather Service issues heat advisories and warnings based on heat index thresholds. Practical example: on a summer day with air temperature of 92°F and relative humidity of 65%, the heat index calculation yields approximately 108°F. This falls in the "Danger" tier (103-124°F), meaning heat cramps and heat exhaustion are likely, and heatstroke is possible with prolonged exposure. At this level, OSHA recommends heavy work for no more than 20 minutes per hour with mandatory 15-minute rest breaks in shade or AC. Edge cases: the NOAA formula is valid only when the temperature is at least 80°F and the relative humidity is at least 40%. Below 80°F, the heat index equals the actual temperature because the humidity effect is negligible. For temperatures below 80°F but above 70°F with very high humidity (90%+), you may still feel uncomfortable, but the standard heat index formula was not designed for this range. For wet-bulb globe temperature (WBGT) measurements used by the military and athletic trainers, add 10°F for sunny conditions and account for wind speed. A reading of 90°F at 60% humidity gives a heat index of ~100°F, but in direct sun with no wind, the WBGT could be 105°F or higher. At 100°F with any humidity above 35%, the heat index exceeds 110°F — dangerously close to the heatstroke threshold.
Frequently Asked Questions
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