Heat Index Calculator
Calculate the apparent temperature based on actual air temperature and relative humidity. The heat index is a measure of how hot it feels when relative humidity is factored in with the actual air temperature.
Heat Index Result
Danger Levels
80°F - 90°F
90°F - 103°F
103°F - 124°F
125°F+
Heat Index Chart
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About the Heat Index
The heat index (HI) is an index that combines air temperature and relative humidity to determine the human-perceived equivalent temperature—how hot it feels. The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate of sweat is reduced, making it more difficult for the body to cool itself and resulting in a higher apparent temperature.
How the Heat Index is Calculated
The heat index is calculated using the following formula:
HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²
Where:
- T is the air temperature in °F
- R is the relative humidity percentage
- c₁ to c₉ are constants
This formula is derived from multiple regression analysis of various temperature and humidity combinations.
Health Effects of High Heat Index
Exposure to high heat index values can lead to several heat-related illnesses:
- Heat cramps: Painful muscle spasms that usually occur during heavy exercise in hot environments
- Heat exhaustion: Symptoms include heavy sweating, weakness, cold/pale/clammy skin, fast/weak pulse, nausea/vomiting, and fainting
- Heat stroke: A life-threatening condition where the body temperature rises above 103°F. Symptoms include hot/red/dry skin, rapid/strong pulse, and possible unconsciousness
Precautions for High Heat Index
When the heat index is high, take these precautions:
- Stay in air-conditioned buildings as much as possible
- Drink plenty of fluids, even if you don't feel thirsty
- Wear lightweight, light-colored, loose-fitting clothing
- Schedule outdoor activities for cooler parts of the day
- Pace yourself when working or exercising in heat
- Never leave children or pets in cars
- Check on at-risk friends, family, and neighbors
Limitations of the Heat Index
The heat index has some limitations:
- It assumes you're in the shade and light wind conditions
- It doesn't account for direct sunlight, which can increase the felt temperature by up to 15°F
- It's less accurate for temperatures below 80°F
- It doesn't factor in personal characteristics like age, weight, or fitness level
- It assumes you're properly hydrated