Work out the heat index from air temperature and humidity with the National Weather Service formula, plus dew point and humidex.
The heat index is the temperature the body feels when humidity is added to the air temperature. At 95 °F (35 °C) and 60% relative humidity the heat index is 113 °F (45 °C), so it feels 18 degrees hotter than the thermometer says. The United States National Weather Service computes it with the Rothfusz regression, an equation fitted to a model of how the body loses heat, using temperature in °F and humidity in percent. Below about 80 °F a simple formula is used instead, and two adjustments apply at very low humidity (under 13%) and at high humidity (over 85%) between 80 and 87 °F. The formula box shows the result for your numbers.
The NWS groups heat index values into four bands: 80 to 90 °F is caution, where fatigue is possible with prolonged exposure; 90 to 103 °F is extreme caution, where heat cramps and heat exhaustion are possible; 103 to 125 °F is danger, where heat exhaustion is likely and heat stroke is possible; and 125 °F or more is extreme danger. In Celsius those limits are about 27, 32, 39.4 and 51.7 °C. The category is shown under the result, and the table lists the heat index and category at other humidity levels so you can see how much humidity matters. This is general information, not medical advice.
The dew point is the temperature at which the air would be saturated. The page uses the Magnus formula with the constants 17.625 and 243.04 °C from Alduchov and Eskridge (1996): at 30 °C and 70% humidity the dew point is 23.9 °C. A dew point above about 20 °C (68 °F) feels muggy to most people. The humidex is the Canadian equivalent of the heat index. It adds the vapour pressure from the dew point to the temperature: 30 °C and 70% gives a humidex of 41, which Environment Canada describes as great discomfort. Two scales exist because the countries built them separately, so the numbers differ.
The heat index assumes shade and a light wind. Standing in full sun can add up to about 15 °F (8 °C) to what the body feels. The regression was fitted for temperatures from 80 °F and humidity of 40% and above, so results outside that range are less reliable, and the page tells you when your numbers are there. The heat index is a guide to weather, not to an individual: age, health, medicines, clothing and fitness all change how a person handles heat. It does not account for working hard in the heat, which is covered by wet-bulb globe temperature guidance from occupational safety bodies.
About 100 °F, which falls in the extreme caution band. At 90 °F and 50% it is 95 °F and at 70% it is 106 °F, so each 10% of humidity adds around 5 degrees at this temperature.
The NWS uses HI = −42.379 + 2.04901523 T + 10.14333127 R − 0.22475541 T R − 0.00683783 T² − 0.05481717 R² + 0.00122874 T² R + 0.00085282 T R² − 0.00000199 T² R², with T in °F and R as a percentage. Adjustments apply below 13% and above 85% humidity.
The NWS calls 103 °F (39.4 °C) and above danger, and 125 °F (51.7 °C) and above extreme danger. 100 °F at 60% humidity gives 129 °F, which is extreme danger.
Use the Magnus formula. At 20 °C and 50% humidity the dew point is 9.3 °C, and at 30 °C and 70% it is 23.9 °C. Type the temperature and humidity and the page shows it in your unit.
The NWS formula is meant for 80 °F (27 °C) and above, and at lower temperatures the simple formula gives a value close to the air temperature. At 70 °F and 50% the heat index is 69 °F.
No. The temperature and humidity stay in this tab and are not stored or sent. Reloading puts the 95 °F example back. The page only counts that a result was shown, without your numbers.