Heat index, often referred to as humiture, measures the perceived rather than the actual temperature: an air temperature of 83°F with a relative humidity of 70% results in an estimated 88°F perceived temperature, because high humidity slows the evaporation of sweat that cools the body.

How is heat index calculated?

The National Weather Service uses the Rothfusz regression of temperature and relative humidity, developed by George Winterling in 1978 and meant to be valid for temperatures of 80°F or higher and relative humidity of 40% or more, with adjustments for very low and very high humidity.

Heat Index Calculator

This calculator estimates the temperature felt by the body as a result of air temperature and relative humidity.

Use Relative Humidity

Air Temperature Relative Humidity %

Use Dew Point Temperature

Air Temperature Dew Point Temperature

What is Heat Index?

Heat index is often referred to as humiture, and is similar to wind chill in its attempt to measure the perceived rather than the actual temperature. For example, an air temperature of 83°F with a relative humidity of 70% would result in an estimated 88°F perceived temperature. This difference in perceived and actual temperature is the result of a mixture of air temperature, relative humidity, and wind speed.

The perception of heat is subjective, and can be affected by various factors such as menopause, pregnancy, and the effects of drugs or withdrawal, as well as differences in hydration, body shape, and metabolism. A higher relative humidity affects normal body cooling by reducing the rate of evaporation of sweat. The human body cools itself through perspiration, where heat is removed from the body as a result of the evaporation of sweat. The lower rate of evaporation subsequently lowers the rate at which the body cools, increasing the perception of heat. This perception of heat is what the heat index seeks to measure, and while it can technically be used indoors, it is most often used in reference to the outside temperature.

How to Calculate Heat Index?

Like the Wind Chill Temperature Index, the heat index used by the National Weather Service (NWS) in the United States is based on many assumptions such as body mass, height, clothing, individual physical activity, blood thickness, and wind speed. As such, depending on how significantly these assumptions vary from the reality of an individual, heat index estimates may not accurately reflect the perceived temperature. The equation used by the NWS to estimate heat index was developed by George Winterling in 1978, and is meant to be valid for temperatures of 80°F or higher, and relative humidity of 40% or more. Below is a chart based on the NWS equation that can be used to estimate temperature and the level of danger associated with varying relative humidity percentages.

Heat index (°F) by air temperature (°F, columns) and relative humidity (%, rows):

	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45	80	82	84	87	89	92	96	100	104	109	114	119	124	130	137	143
50	81	83	85	88	91	95	99	103	108	113	118	124	131	137	144	152
55	81	84	86	89	93	97	101	106	112	117	124	130	137	145	153	161
60	82	84	88	91	95	100	105	110	116	123	129	137	145	153	162	171
65	82	85	89	93	98	103	108	114	121	128	136	144	153	162	172	182
70	83	86	90	95	100	106	112	119	126	134	143	152	161	172	182	194
75	84	88	92	97	103	109	116	124	132	141	150	160	171	182	193	206
80	84	89	94	100	106	113	121	129	138	148	158	169	181	193	205	219
85	85	90	96	102	110	117	126	135	145	155	167	179	191	204	218	233
90	86	92	98	105	113	122	131	141	152	164	176	189	202	216	231	247
95	88	94	101	109	117	127	137	148	160	172	185	199	214	229	245	262
100	89	96	104	112	121	132	143	155	168	181	195	210	226	243	260	278

Caution Extreme Caution Danger Extreme Danger

Potential effects of heat index

As described above, the heat index is the temperature equivalent perceived by humans as a result of air temperature, relative humidity, and wind speed. This temperature can have potentially severe medical effects. Under high air temperature and humidity (high heat index) conditions, perspiration is hindered due to reduced evaporation as a result of high humidity. Sweat is the human body's physiological response to high temperatures, and is an attempt to lower body temperature through the evaporation of sweat. When this is hindered, overheating and dehydration can occur, with varying severity. Below is a table indicating possible complications at varying levels of heat index values, obtained from Wikipedia.

Effects of the heat index

Celsius	Fahrenheit	Notes
27-32°C	80-90°F	Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
32-41°C	90-105°F	Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
41-54°C	105-130°F	Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
Over 54°C	Over 130°F	Extreme danger: heat stroke is imminent.

Note that exposure to full sunshine can increase heat index values by up to 14°F. Heat index values are particularly important for children. Young children are generally in more danger due to factors including larger skin surface relative to their small bodies, higher heat production as a result of exercise, and typically sweating less than adults. Also, children are often less aware than adults of the need to rest and re-hydrate.

Thirst is a late sign of dehydration, and it is important to remain hydrated, particularly before, during, and after outdoor activities, especially those involving heavy physical exertion. In addition to children, people with certain conditions, including obesity, diabetes, heart disease, cystic fibrosis, and mental retardation are at greater risk of overheating and dehydration.

Frequently Asked Questions

Why does the result differ from the air temperature so much?

When humidity is high, sweat evaporates slowly, so the body cools less efficiently and the perceived temperature (the heat index) can be far above the measured air temperature.

Can I use dew point instead of relative humidity?

Yes. The second calculator converts the dew point and air temperature into relative humidity first, then applies the same heat index equation.