03 / What's happening to your body?
Why does humidity make heat feel worse?
Your body produces sweat to help regulate its temperature. But sweating alone doesn't cool you very much. Evaporation is the key. When sweat evaporates from your skin, it carries heat away.
When relative humidity is high, the surrounding air contains more water vapor relative to what it could hold at that temperature. This reduces the potential for evaporation. Your body can struggle to cool efficiently, making the same air temperature feel more oppressive.
Scenario A
Hot & relatively dry
Sweat evaporates more readily
Lower humidity generally supports more effective evaporative cooling, although high temperatures can still be dangerous.
Scenario B
Hot & very humid
Sweat evaporates less efficiently
Higher humidity slows evaporative cooling, increasing the potential for heat stress during exposure or activity.
The surprising part: A higher humidity reading doesn't increase the actual air temperature. It changes how effectively your body can cool itself. That's why air temperature and heat index are not interchangeable.
04 / Know the warning signs
Heat index risk levels explained
The National Weather Service uses heat index categories to communicate potential risks associated with hot, humid conditions. These ranges are general guidance, not a personal safety guarantee.
80–89°F / approximately 27–32°C
Caution
Fatigue may occur with prolonged exposure or physical activity. Begin paying attention to hydration, rest, and heat exposure.
90–102°F / approximately 32–39°C
Extreme Caution
Heat cramps and heat exhaustion become possible, especially with prolonged exposure or strenuous activity.
103–124°F / approximately 39–51°C
Danger
Heat cramps and heat exhaustion are likely with continued exposure, and heat stroke becomes a possibility.
125°F+ / approximately 52°C+
Extreme Danger
Heat stroke is highly likely with continued exposure. Avoid strenuous activity and seek effective cooling.
Important context: These categories are based on the heat index, not the ordinary air temperature. Risk also depends on age, health, acclimatization, sun exposure, clothing, workload, and duration of exposure. Dangerous heat illness can occur outside these categories.
05 / Behind the calculation
How is the heat index calculated?
The heat index is not calculated by simply adding temperature and humidity. The relationship is nonlinear: the influence of humidity changes depending on the air temperature.
UtilityGlen follows the National Weather Service heat index method. It begins with a simpler preliminary estimate and uses the Rothfusz regression when the preliminary heat index reaches 80°F. The method also applies adjustments for certain combinations of unusually low or high humidity and temperature.
Calculation inputs
Two measurements. One apparent temperature.
Air Temperature
Degrees Celsius or Fahrenheit
Relative Humidity
Percentage from 0 to 100
Calculated output
Heat Index / Feels-Like Temperature
Estimated apparent heat under shaded, light-wind conditions
What is the Rothfusz regression?
It's an empirical polynomial equation used by the NWS to estimate heat index from temperature in Fahrenheit and relative humidity. It includes squared and interaction terms, allowing for a nonlinear response.
When is the formula less reliable?
The standard heat-index model is intended mainly for hot-weather conditions. Values outside its typical application range should be interpreted cautiously, especially when judging real-world heat exposure.
For the official methodology and heat index background, see the
National Weather Service heat index explanation .
06 / Real-world context
Why does the heat index matter?
Heat index information is useful wherever people need to interpret hot, humid weather. It helps provide context, although it cannot replace the complete assessment of outdoor heat stress.
Exercise and recreation
Outdoor workouts
Running, cycling, hiking, and outdoor sports increase the amount of heat the body produces. Humidity can limit evaporative cooling, making conditions harder to tolerate during physical activity.
Outdoor occupations
Workplace awareness
Construction, farming, landscaping, and other outdoor work can involve substantial heat exposure. Heat index is one useful indicator, but proper workplace assessment also considers exertion, clothing, radiant heat, and acclimatization.
Daily comfort
Understanding weather reports
Comparing air temperature with the heat index helps explain why two days with the same thermometer reading can feel very different, particularly in humid climates.
Science and education
Learning meteorology
Teachers, students, and weather enthusiasts can experiment with temperature and humidity values to explore the relationship between atmospheric moisture and perceived heat.
07 / Don't confuse the numbers
Heat index vs. temperature vs. humidity
These weather measurements are related, but each describes something different. Understanding the distinction helps you interpret weather reports more accurately.
Comparison of air temperature, relative humidity, heat index, and wet-bulb temperature
| Air temperature |
The measured temperature of the surrounding air |
°C or °F |
| Relative humidity |
How close air is to water-vapor saturation at its temperature |
% |
| Heat index |
Estimated apparent heat combining air temperature and humidity |
°C or °F |
| Wet-bulb temperature |
Temperature reflecting evaporative cooling under specified conditions |
°C or °F |
Heat index is not the same as wet-bulb temperature
Both are relevant to heat and moisture, but they are different measures. Heat index estimates perceived heat, whereas wet-bulb temperature represents evaporative cooling conditions. Wet-bulb globe temperature (WBGT) is another measure that incorporates additional environmental factors and is often more useful for occupational or athletic heat-stress assessment.