Calculate heat index from temperature and humidity with heat exhaustion risk levels.
Hot air and humid air can share a thermometer reading but place very different demands on the body. This calculator takes an air temperature and relative humidity and returns the heat index, an apparent temperature, plus a heat-illness risk level. Enter humidity as a percentage: fifty percent is 50, not 0.50. The result often surprises users because increasing humidity can raise the apparent temperature sharply without changing the measured air temperature. It describes combined conditions, not a new thermometer reading or a personal body-temperature estimate.
A concrete input and expected output from the current implementation.
Input
Air temperature: 90 degrees F; relative humidity: 50%
Expected output
Heat index: 94.6 degrees F, or about 95 degrees F; risk level: Extreme Caution
The standard heat-index equation gives 94.5969 degrees F for these inputs, which rounds to 94.6 degrees F at one decimal place or 95 degrees F at whole-degree precision. That apparent temperature is in the Extreme Caution range.
Start with the measured or forecast air temperature and relative humidity, not dew point. The heat-index relationship combines both values because humidity affects how readily perspiration can evaporate. It reports an equivalent apparent temperature for that pair of conditions. Temperature and humidity are not averaged, and a ten-point humidity change does not add a fixed number of degrees in every case.
After determining the apparent temperature, the calculator assigns a heat-risk band such as Extreme Caution. The band summarizes the increasing potential for heat-related illness during prolonged exposure or strenuous activity. It does not determine a safe exposure time, prescribe water intake, estimate a worker's core temperature, or decide whether a particular person can safely continue an activity.
High humidity slows the evaporation of perspiration, reducing one of the body's main cooling mechanisms. The same measured temperature can therefore correspond to a higher apparent temperature and greater heat stress. No, the result does not mean the surrounding air has warmed; it describes how the temperature-humidity combination is experienced under the heat-index assumptions.
No. Dew point and relative humidity both describe moisture, but they are not interchangeable numeric inputs. Relative humidity depends on air temperature, so a dew-point value must first be converted using the accompanying temperature. Entering a dew point such as 70 as though it meant 70% relative humidity can produce a substantially different heat index.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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