What heat index is dangerous?

NWS rates 103°F to 124°F (39°C to 51°C) as danger, where heat cramps and heat exhaustion are likely and heat stroke is possible with prolonged exposure, and 125°F (52°C) and above as extreme danger. Even the 90–102°F extreme caution band can cause heat illness with exertion.

Why can the heat index be lower than the temperature?

In very dry air, sweat evaporates efficiently and the body cools well, so hot, dry conditions can feel slightly cooler than the air temperature. That is why the equation can return a value a degree or two below the thermometer at low humidity.

Does the heat index include sunshine?

No. It assumes shade and a light breeze. NWS notes that direct sunshine can raise the heat index by up to 15°F. For sun and wind as well as humidity, occupational and sports guidance uses the wet-bulb globe temperature (WBGT).

Why does this calculator not match the NWS chart exactly?

The printed NWS chart comes from Steadman’s original tables, while calculators, including NWS’s own, use the Rothfusz regression fitted to those tables. They agree to within about 1°F over most of the chart, and the adjustments for very dry or very humid air can move a cell by a degree or two.

Is the heat index the same as “feels like”?

At 80°F and above, yes: the NWS feels-like temperature is the heat index. In the cold it is the wind chill, and in between it is the air temperature. The feels-like calculator applies that rule for you.

Heat stress · NWS method

Heat Index Calculator

The heat index combines air temperature and relative humidity into how hot it feels to a person in the shade. Humid air slows the evaporation of sweat, so it feels hotter than the thermometer reads.

Formula

Simple estimate first: HI = 0.5·[T + 61 + (T − 68)·1.2 + RH·0.094]. If that reaches 80°F, the regression below is used instead.
HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²
If RH < 13% and 80 ≤ T ≤ 112°F: subtract [(13 − RH)/4]·√{[17 − |T − 95|]/17}.
If RH > 85% and 80 ≤ T ≤ 87°F: add [(RH − 85)/10]·[(87 − T)/5].

Source: Rothfusz (1990) regression of Steadman’s apparent temperature tables, with the adjustments published by the NWS Weather Prediction Center. T in °F, RH in percent.

Valid range

The NWS heat index chart covers 80°F to 110°F (27°C to 43°C) and 40% to 100% humidity. The regression fits Steadman’s tables to within about ±1.3°F there.
Below 80°F the calculator uses the simple formula, as NWS does, and the result stays close to the air temperature. It is flagged, because the heat index is not meant for mild weather.
Above 110°F the regression is being extrapolated and grows quickly. The calculator still shows the value but flags it as a rough indication only.
The heat index assumes shade and light wind. NWS notes that full sunshine can raise it by up to 15°F.

Reading the heat index

Sweat cools you only when it evaporates. The more water vapor already in the air, the slower that happens, so a humid 92°F day can feel like 105°F while a dry one feels close to the thermometer. The heat index puts a number on that.

NWS groups heat index values into four risk bands: caution (80–89°F), extreme caution (90–102°F), danger (103–124°F) and extreme danger (125°F and above). Local heat advisories and warnings are issued against criteria set by each forecast office, so the same heat index can trigger an advisory in one city and not another.

In very dry air the regression slightly overstates the heat index, and between 80°F and 87°F in very humid air it slightly understates it. NWS publishes two small adjustments for those corners, and both are applied here.

Heat index chart (°F)

Generated from the same equation as the calculator above.

Heat index chart (°F)
Humidity (%) / Temperature (°F)	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
50%	81	83	85	88	91	95	99	103	108	113	118	124	131
55%	81	84	86	89	93	97	101	106	112	117	124	130
60%	82	84	88	91	95	100	105	110	116	123	129	137
65%	82	85	89	93	98	103	108	114	121	128	136
70%	83	86	90	95	100	106	112	119	126	134
75%	84	88	92	97	103	109	116	124	132
80%	84	89	94	100	106	113	121	129
85%	85	90	96	102	110	117	126	135
90%	86	92	98	105	113	122	131
95%	88	94	101	109	117	127	137
100%	89	96	104	112	121	132

Caution, 80–89°F (27–32°C)
Extreme caution, 90–102°F (32–39°C)
Danger, 103–124°F (39–51°C)
Extreme danger, 125°F+ (52°C+)

Blank cells would be above 137°F, far beyond the data the regression was fitted to. The printed NWS chart comes from Steadman's original tables, so a few cells there differ from the equation by a degree or two.

Heat index chart (°C)

Generated from the same equation as the calculator above.

Heat index chart (°C)
Humidity (%) / Temperature (°C)	27°	29°	31°	33°	35°	37°	39°	41°	43°
40%	27	29	31	34	37	41	46	51	57
50%	27	30	33	36	41	46	52	58
60%	28	31	35	40	45	51
70%	29	33	38	43	50	58
80%	30	35	41	48	57
90%	31	37	45	54
100%	33	40	49

Heat index questions

What heat index is dangerous?: NWS rates 103°F to 124°F (39°C to 51°C) as danger, where heat cramps and heat exhaustion are likely and heat stroke is possible with prolonged exposure, and 125°F (52°C) and above as extreme danger. Even the 90–102°F extreme caution band can cause heat illness with exertion.
Why can the heat index be lower than the temperature?: In very dry air, sweat evaporates efficiently and the body cools well, so hot, dry conditions can feel slightly cooler than the air temperature. That is why the equation can return a value a degree or two below the thermometer at low humidity.
Does the heat index include sunshine?: No. It assumes shade and a light breeze. NWS notes that direct sunshine can raise the heat index by up to 15°F. For sun and wind as well as humidity, occupational and sports guidance uses the wet-bulb globe temperature (WBGT).
Why does this calculator not match the NWS chart exactly?: The printed NWS chart comes from Steadman’s original tables, while calculators, including NWS’s own, use the Rothfusz regression fitted to those tables. They agree to within about 1°F over most of the chart, and the adjustments for very dry or very humid air can move a cell by a degree or two.
Is the heat index the same as “feels like”?: At 80°F and above, yes: the NWS feels-like temperature is the heat index. In the cold it is the wind chill, and in between it is the air temperature. The feels-like calculator applies that rule for you.

Heat Index Calculator