113.7
°F
45.4
°C
3
1-4
18.7
°F
113.7
°F
45.4
°C
3
1-4
18.7
°F
The heat index, sometimes called the "apparent temperature" or "feels-like temperature," combines air temperature and relative humidity to express how hot it actually feels to the human body. When humidity is high, sweat evaporates more slowly, reducing the body's primary cooling mechanism and making the effective temperature significantly higher than the thermometer reading.
The U.S. National Weather Service (NWS) uses the heat index as a key metric for issuing heat advisories and warnings. At a measured temperature of 95 °F with 55% relative humidity, for example, the heat index climbs to approximately 110 °F — well into the danger zone for heat-related illness. Understanding these values is crucial for outdoor workers, athletes, event planners, and public health officials.
Our Heat Index Calculator implements the Rothfusz regression equation, the standard polynomial used by the NWS. Enter the air temperature (in °F) and relative humidity, and the tool instantly returns the apparent temperature in both Fahrenheit and Celsius, along with the NWS danger category. This helps you make informed decisions about outdoor activity, hydration needs, and heat safety protocols.
The calculator uses the Rothfusz regression equation, a multiple regression fit to the Steadman heat index model:
$$HI = -42.379 + 2.049T + 10.143RH - 0.2248T \cdot RH - 0.00684T^2 - 0.0548RH^2 + 0.00123T^2 RH + 0.000853T \cdot RH^2 - 0.00000199T^2 RH^2$$
where T is the air temperature in °F and RH is the relative humidity in percent. This equation is valid for temperatures at or above 80 °F and provides a close approximation to the original Steadman tables.
The NWS defines four danger categories based on the heat index value: Caution (80–90 °F), Extreme Caution (90–103 °F), Danger (103–125 °F), and Extreme Danger (above 125 °F). Each level corresponds to increasing risk of heat cramps, heat exhaustion, and heat stroke.
The Celsius conversion applies the standard formula: $$T_C = (HI_F - 32) \times \frac{5}{9}$$
A heat index below 90 °F (Caution) means fatigue is possible with prolonged exposure. Between 90–103 °F (Extreme Caution), heat cramps and heat exhaustion become likely. From 103–125 °F (Danger), heat stroke is probable with continued exposure. Above 125 °F (Extreme Danger), heat stroke is highly likely. Always combine the heat index with exposure duration and individual risk factors when assessing danger.
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At 95 °F and 55% humidity, the heat index reaches ~110 °F (Danger category). Outdoor activity should be limited and hydration is critical.
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Even at a moderate 90 °F, very high humidity (85%) pushes the heat index to ~115 °F, demonstrating the powerful effect of moisture on perceived heat.
The heat index is the apparent temperature that accounts for the combined effects of air temperature and relative humidity on the human body. It is calculated using the Rothfusz regression equation, a polynomial with nine terms that approximates the original Steadman model. The NWS adopted this equation as its operational standard because it closely reproduces the Steadman lookup tables across the valid range.
Humidity slows the evaporation of sweat, which is the body's primary cooling mechanism. When the air is already saturated with moisture, sweat stays on the skin rather than evaporating, reducing heat loss. The body's core temperature rises, and you perceive the environment as hotter than the actual air temperature. This physiological effect is what the heat index quantifies.
The Rothfusz regression equation is designed for temperatures at or above 80 °F. Below this threshold, the heat index is essentially equal to the air temperature because humidity has minimal physiological impact at lower temperatures. The NWS does not publish heat index values below 80 °F for this reason.
The heat index is a perceived temperature based on empirical regression against human comfort data, while the wet-bulb temperature is a physical measurement obtained by wrapping a thermometer in a wet cloth and allowing evaporative cooling. Wet-bulb temperature directly measures the atmosphere's evaporative cooling potential and is used in industrial hygiene (WBGT), whereas the heat index is oriented toward public weather communication.
OSHA recommends enhanced precautions when the heat index exceeds 91 °F (Extreme Caution). Above 103 °F (Danger), NIOSH guidelines recommend limiting heavy physical labor, providing mandatory rest breaks, and ensuring immediate access to water and shade. Above 115 °F, only essential emergency work should continue. Specific thresholds vary by organization and jurisdiction.
Wind can increase sweat evaporation and provide some cooling, but the standard heat index formula does not account for wind speed. The heat index assumes shady, light-wind conditions. Direct sunlight can add up to 15 °F to the perceived heat index, while strong wind may partially offset it. For a wind-adjusted metric, the wet-bulb globe temperature (WBGT) is more appropriate.
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The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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