What is the Heat Index?
The Heat Index (HI), often referred to as humiture, is an index that combines air temperature and relative humidity to determine the human-perceived equivalent temperature. It functions similarly to wind chill, but instead of measuring how cold the wind makes it feel, the heat index attempts to measure how hot the weather actually feels to the human body.
For example, if the air temperature is 83°F with a relative humidity of 70%, the air will feel closer to an estimated 88°F. This difference in actual versus perceived temperature occurs because high relative humidity restricts the evaporation of sweat, making it harder for the body to cool itself efficiently.
How the Human Body Cools Itself
The human body regulates its internal temperature by sweating (perspiration). As sweat evaporates off the skin, it absorbs body heat and carries it away, thereby cooling you down. However, the rate of evaporation is heavily dependent on the moisture already present in the air—the relative humidity.
When humidity is high, the air is already saturated with water vapor, meaning sweat evaporates much slower. Consequently, heat is not removed from the body as quickly, causing a person to feel noticeably hotter than the actual measured air temperature. The heat index provides a quantifiable measure of this perceived heat.
How to Use This Heat Index Calculator
Our tool allows you to estimate the Heat Index using two main methods depending on the data you have available:
- Using Relative Humidity: Enter the actual air temperature and the current relative humidity percentage. This is the most common method as standard weather reports typically provide humidity as a percentage.
- Using Dew Point: If you know the dew point instead of the relative humidity percentage, simply select the "Dew Point" calculation method. Enter the air temperature and the dew point temperature. The calculator will automatically convert the dew point into relative humidity to run the heat index algorithm.
The Heat Index Formula
The standard Heat Index formula utilized by the United States National Weather Service (NWS) is derived from the work of George Winterling in 1978 and later refined by meteorologists. The mathematical model considers numerous assumptions involving body mass, physical activity, clothing, and wind speed.
HI = -42.379 + 2.04901523(T) + 10.14333127(RH) - 0.22475541(T)(RH) - 0.00683783(T²) - 0.05481717(RH²) + 0.00122874(T²)(RH) + 0.00085282(T)(RH²) - 0.00000199(T²)(RH²)
Where:
HI = Heat Index (expressed as an apparent temperature in degrees Fahrenheit)
T = Ambient dry-bulb temperature (in degrees Fahrenheit)
RH = Relative humidity (percentage value between 0 and 100)
Note: The NWS formula is scientifically valid only for air temperatures of 80°F (27°C) or higher and relative humidity levels of 40% or more. For temperatures below 80°F, a simpler apparent temperature formula is often used since the body does not perceive a drastically higher temperature in those ranges.
Potential Health Effects of High Heat Index
Understanding the heat index is critical for public safety. Prolonged exposure to high heat indices—especially combined with physical activity—can lead to severe medical emergencies such as heat exhaustion and heat stroke.
| Temperature Range | Danger Level | Potential Health Effects |
|---|---|---|
| 80°F - 90°F (27°C - 32°C) | Caution | Fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps. |
| 90°F - 105°F (32°C - 41°C) | Extreme Caution | Heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke. |
| 105°F - 130°F (41°C - 54°C) | Danger | Heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity. |
| Over 130°F (Over 54°C) | Extreme Danger | Heat stroke is imminent and highly dangerous. |
Be aware that exposure to direct, full sunshine can increase heat index values by up to 15°F. Children, the elderly, and individuals with specific health conditions (such as obesity, diabetes, and heart disease) are at a significantly higher risk for heat-related illnesses and dehydration.
Frequently Asked Questions
Humidity makes the air feel hotter because the human body relies on the evaporation of sweat to regulate internal temperature. When the relative humidity is high, the air is already holding a lot of moisture, which slows down the rate at which sweat evaporates off your skin. Without efficient evaporation, the body retains more heat, making the surrounding environment feel drastically hotter than the actual measured air temperature.
Yes, under specific conditions. If the air temperature is high but the relative humidity is exceptionally low (often below 15-20%), the rapid evaporation of sweat can actually cool the body efficiently enough that the perceived temperature (heat index) is slightly lower than the actual air temperature. However, this is relatively rare outside of arid desert climates.
Relative Humidity is a ratio (expressed as a percentage) of the current absolute humidity to the highest possible absolute humidity at the current air temperature. It tells you how saturated the air is. Dew Point is the absolute temperature to which air must be cooled to reach 100% saturation (where water vapor condenses into dew). Meteorologists often prefer dew point because it gives a more direct measurement of the actual moisture content in the air regardless of the temperature fluctuations.
The National Weather Service typically issues heat advisories when the heat index is expected to exceed 100°F - 105°F for at least two days. A heat index above 105°F is generally considered "Dangerous," where heat exhaustion is likely and heat stroke becomes probable with physical activity. Heat index values over 130°F are deemed "Extreme Danger," making heat stroke imminent.
The standard Heat Index formula (Rothfusz regression) assumes a light wind breeze of approximately 5 knots (about 5.75 mph). If the wind is completely still, the perceived temperature may actually feel slightly hotter than the calculated heat index. Conversely, strong winds in extremely hot, dry conditions can sometimes increase heat stress by adding convective heat to the body, contrary to standard cooling breezes.