Heat Index Calculator

This calculator estimates the temperature the body feels as a result of air temperature and relative humidity. Enter a temperature and a humidity reading, or a temperature and a dew point if that is what you have, and it returns the heat index in Fahrenheit, Celsius and Kelvin with the risk category attached. At 30 °C and 70 percent humidity the heat index is 35 °C, which is 95 °F or 308 K.

What the heat index is

The heat index, sometimes called humiture, is an attempt to measure perceived rather than actual temperature, in the same way wind chill does at the other end of the year. An air temperature of 83 °F at 70 percent relative humidity feels like about 88 °F. The gap between what the thermometer reads and what the body registers comes from the combination of air temperature, relative humidity and wind speed.

The reason humidity matters is evaporation. The body cools itself by sweating, and the cooling happens when the sweat evaporates, not when it appears. Evaporation moves heat off the skin and into the air. Air that is already carrying a lot of water vapour accepts less, so the sweat evaporates more slowly, less heat leaves, and the same air temperature feels hotter. At 100 percent humidity sweat barely evaporates at all, and the body loses its main cooling route.

The perception of heat is also personal. Hydration, body shape and metabolism all move it, and so do menopause, pregnancy, and the effects of some drugs or of withdrawal. The index can technically be used indoors, but it is almost always quoted for outdoor conditions.

How the heat index is calculated

Like the wind chill index, the heat index used by the National Weather Service rests on a set of assumptions: a particular body mass and height, clothing, level of physical activity, blood thickness and wind speed. The further your situation is from those assumptions, the less the number describes you.

The underlying model is Robert Steadman's 1979 work on apparent temperature, published as a set of tables. Those tables are awkward to compute from, so in 1978 George Winterling produced the humiture concept and in 1990 Lans Rothfusz fitted a polynomial regression to Steadman's results. That regression is what the NWS uses and what this calculator implements. Rothfusz quotes an error of about ±1.3 °F against the tables he fitted, which is worth knowing: a cell here can sit a degree away from a printed NOAA chart without either being wrong.

The equation is meant for temperatures of 80 °F or higher and relative humidity of 40 percent or more. Below that the calculator falls back to a simpler average, which is what the NWS procedure specifies, because the regression misbehaves outside the range it was fitted to. Two corrections are applied at the edges: below 13 percent humidity the figure is reduced slightly, and above 85 percent humidity in the 80 to 87 °F band it is increased.

The heat index chart

This is the National Weather Service grid, with air temperature across the top and relative humidity down the side. Every cell is produced by the same function the calculator above uses, so the two always agree.

The pattern in the grid is the useful part. Follow a single row and the heat index climbs steadily with temperature, which is unsurprising. Follow a single column and it climbs with humidity, which is the part people underestimate. At 90 °F the heat index is 91 °F at 40 percent humidity and 106 °F at 70 percent. The thermometer has not moved at all, and the felt temperature has gone up fifteen degrees.

That is why a dry 105 °F afternoon at 40 percent humidity, at a heat index of 121 °F, is less dangerous than 100 °F at 60 percent, which reaches 129 °F. Air temperature alone ranks those two the wrong way round.

Working from a dew point instead

Weather reports often give a dew point rather than a relative humidity, and the second calculator on this page takes it directly. The dew point is the temperature the air would have to cool to before water condensed out of it, so it measures the absolute amount of moisture in the air rather than a percentage of capacity.

That makes it the better number for comparing days. Relative humidity of 70 percent means something different in the morning than in the afternoon, because warm air holds more water, so the percentage falls through the day even when nothing has changed. A dew point of 22 °C is the same amount of moisture whenever you read it. At an air temperature of 30 °C, that dew point works out at 62 percent relative humidity and a heat index of 33 °C.

As a rough guide, a dew point below 13 °C feels comfortable, in the high teens it feels sticky, and above 21 °C most people find it oppressive.

Effects of the heat index

Under high heat and humidity, perspiration is hindered, and since sweating is the body's main response to high temperature the result is overheating and dehydration of varying severity. The table below lists the complications usually associated with each band.

Two adjustments matter in practice. Full sunshine can raise the heat index by as much as 14 °F over a shaded reading, so a chart value taken in the shade understates what somebody standing in the open is experiencing. And these figures assume light wind; a strong hot wind can make things worse rather than better, since air hotter than skin adds heat instead of removing it.

Who is most at risk

Heat index values matter particularly for children. Young children have a larger skin surface relative to their small bodies, produce more heat for a given amount of exercise, and typically sweat less than adults. They are also less likely than adults to notice they need to stop and rehydrate.

Thirst is a late signal of dehydration, so the advice is to drink before, during and after outdoor activity rather than waiting to feel it. Beyond children, people with obesity, diabetes, heart disease, cystic fibrosis or cognitive impairment are at greater risk of overheating and dehydration. So are people on medication that reduces sweating or affects fluid balance, and anyone not yet acclimatised, since it takes a week or two of exposure before the body starts sweating earlier and more efficiently.

The three conditions to recognise, in order of severity, are heat cramps, heat exhaustion and heat stroke. Cramps are painful but not dangerous. Heat exhaustion brings heavy sweating, weakness, nausea and cool clammy skin, and calls for shade, fluids and cooling. Heat stroke is a medical emergency: the core temperature passes about 40 °C, sweating often stops, the skin turns hot and dry, and confusion sets in. It needs emergency help and aggressive cooling immediately.

The limit of what a body can handle

There is a hard physical ceiling behind all of this. Evaporative cooling stops working entirely once the wet-bulb temperature reaches about 35 °C, which is 95 °F, because at that point the air can take no more moisture and sweat cannot evaporate at any rate. A healthy person at rest in the shade with unlimited water cannot survive extended exposure past that, since the body has no remaining way to shed the heat it generates. Normal core temperature is 37 °C, or 98.6 °F, and the margin above it is small.

Wet-bulb readings that high are still rare and brief, but they have been recorded, and they are the reason meteorologists watch dew point rather than temperature alone during heat waves.

Reading the results on this page

The panel gives the heat index in the unit you entered, with the other two beside it, the risk category, and how many degrees hotter the conditions feel than the air temperature. The coloured banner underneath repeats the category with the specific risk attached.