Science & Lab Tools
Heat Index Calculator
Calculate how hot it really feels by combining temperature and humidity levels
Enter temperature and humidity to calculate the heat index
Related to Heat Index Calculator
The Heat Index Calculator combines air temperature and relative humidity to determine how hot it actually feels to the human body. This "feels-like" temperature is crucial because the human body cools itself through perspiration, and humidity affects this cooling process. When humidity is high, sweat evaporates more slowly, making it feel hotter than the actual air temperature.
Calculation Method
The calculator uses the Rothfusz regression formula, which is the standard equation used by the National Weather Service. This formula takes into account the complex relationship between temperature and humidity, including various coefficients determined through extensive meteorological research. The calculation includes adjustments for extreme conditions, such as very low humidity or high temperature-humidity combinations.
Temperature Ranges
The heat index is most accurate at temperatures above 80°F (27°C) and relative humidity above 40%. Below these thresholds, the heat index is generally close to the actual air temperature. The calculator includes special adjustments for extreme conditions to maintain accuracy across a wide range of temperatures and humidity levels.
The heat index value provides crucial information about potential heat-related health risks. Understanding these risk levels is essential for making informed decisions about outdoor activities and taking appropriate precautions.
Risk Levels Explained
- • Below 80°F (27°C): Safe conditions for most activities
- • 80-90°F (27-32°C): Caution - Fatigue possible with prolonged exposure
- • 91-103°F (33-39°C): Extreme Caution - Heat exhaustion possible
- • 104-125°F (40-52°C): Danger - Heat cramps or heat exhaustion likely
- • Above 126°F (52°C): Extreme Danger - Heat stroke highly likely
Recommended Actions
As the heat index rises, take appropriate precautions such as limiting outdoor activities, staying hydrated, wearing light clothing, and seeking air-conditioned environments. Pay special attention to vulnerable individuals, including elderly people, children, and those with chronic conditions.
1. What is the heat index?
The heat index, also known as the "apparent temperature" or "feels like" temperature, combines air temperature and relative humidity to determine how hot it actually feels to the human body. It's particularly important because it accounts for the body's difficulty in cooling itself through perspiration when humidity is high.
2. Why does humidity affect how hot it feels?
Humidity affects the rate at which sweat can evaporate from your skin, which is the body's primary cooling mechanism. When humidity is high, sweat evaporates more slowly, reducing the cooling effect and making it feel hotter than the actual air temperature.
3. When should I be most concerned about the heat index?
You should be particularly cautious when the heat index exceeds 90°F (32°C), especially if you're planning outdoor activities, working outside, or dealing with vulnerable populations. The risk of heat-related illness increases significantly as the heat index rises.
4. How accurate is the heat index calculation?
The heat index calculation is most accurate for temperatures above 80°F (27°C) and relative humidity above 40%. The formula includes adjustments for extreme conditions to maintain accuracy. However, it's important to note that local conditions such as wind, sun exposure, and physical activity can affect how hot it actually feels.
5. What is the scientific source for this calculator?
This calculator uses the Rothfusz regression formula, which was developed by Lans P. Rothfusz and published by the National Weather Service (NWS). The formula is derived from the original work by R.G. Steadman (1979) and is the official equation used by the NWS to calculate heat index. The formula incorporates extensive research data on human physiology and heat transfer under various atmospheric conditions. The risk level categories are based on official NWS guidelines and have been validated through epidemiological studies of heat-related illnesses.