Apparent Temperature (Heat Index) Calculator
An objective utility for calculating the temperature felt by the human body in hot and humid conditions.
Calculator Tool
Use the sliders or input boxes below to specify the air temperature in Fahrenheit and the relative humidity percentage. The result displays the calculated Heat Index, which is the “feels like” temperature.
Definition and Meteorological Formula
The Heat Index (HI) is a measure of apparent temperature developed to quantify the combined effect of air temperature and relative humidity on the human body. Because the human body cools itself primarily through the evaporation of sweat, high humidity significantly impedes this process, making the environment feel substantially hotter than the measured air temperature. The HI calculation provides a more accurate metric of the thermal stress experienced.
The calculation is based on a multiple regression equation developed by the U.S. National Weather Service (NWS) in 1984, derived from work by Steadman (1984). This formula is complex and involves nine coefficients (C1 through C9) to accurately model the non-linear relationship between temperature and humidity. The regression equation, which is only strictly valid for temperatures above 80°F and relative humidity above 40%, is given by:
HI = C1 + C2T + C3R + C4TR + C5T2 + C6R2 + C7T2R + C8TR2 + C9T2R2
Where T is the temperature in degrees Fahrenheit and R is the relative humidity percentage. The coefficients (C1 through C9) are constants defined by the NWS.
Note: At lower temperatures (below 80°F), the Heat Index generally defaults to the air temperature, as humidity has a less pronounced effect on evaporative cooling when the temperature gradient is lower.
Heat Risk Categories and Health Guidance
Meteorological agencies use the calculated Heat Index to issue public health warnings, as prolonged exposure to high apparent temperatures can lead to severe heat-related illnesses. The table below outlines the risk categories and associated physiological effects, which serve as a critical guide for outdoor activity planning.
| Heat Index (°F) | Risk Category | Associated Health Risk (Hidden on Mobile) |
|---|---|---|
| ≥ 126 | Extreme Danger | Heat stroke highly likely. Immediate measures required to prevent heat-related mortality. |
| 103 – 125 | Danger | Heat cramps or heat exhaustion likely; heat stroke possible with prolonged exposure. |
| 91 – 102 | Extreme Caution | Heat cramps and heat exhaustion possible with prolonged exposure and physical activity. |
| 80 – 90 | Caution | Fatigue possible. Strenuous outdoor activity should be limited. |
| < 80 | Safe / Normal | Conditions generally safe. The Heat Index is equivalent to the air temperature. |
Related Factors and Usage Notes
It is important to understand that the Heat Index is calculated assuming conditions in the shade. Direct exposure to intense sunlight can effectively increase the perceived temperature by as much as 15°F. Wind speed is another factor; while not included in the standard HI formula, light winds can increase cooling by promoting sweat evaporation, whereas strong, hot winds can add to the heat load.
For cold conditions, the opposite effect is calculated using the Wind Chill Factor, which quantifies how cold it feels when wind velocity is factored into the actual air temperature. While the Heat Index is critical in summer months, the Wind Chill Factor serves a parallel purpose in winter, warning against the risk of frostbite and hypothermia.
Accuracy Limits: The complex regression formula is mathematically most accurate within a defined range. Using inputs outside of 80°F to 112°F for temperature, and 40% to 100% for humidity, may result in slightly higher calculation error margins, although the relative risk assessment remains valid.
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