Feels-Like Temperature: Heat Index, Wind Chill, Dew Point

Three numbers, three questions

A thermometer measures the air. Your body feels more than that. On a humid summer day sweat evaporates slowly and you feel hotter. On a windy winter day the wind pulls heat off your skin and you feel colder. The National Weather Service (NWS) has a formula for each case and describes both as what the temperature feels like.

The heat index and the wind chill never overlap. One needs heat, the other cold. The dew point is a different kind of number: it describes the air, not your body, and works in any season.

How the heat index is calculated

NWS Amarillo describes the heat index, also called the apparent temperature, as what the temperature feels like to the human body when relative humidity is combined with the air temperature. The NWS Weather Prediction Center publishes the method in three steps, all in °F and percent. The heat index calculator runs exactly these steps.

1. Compute a simple formula based on Steadman's results. If it comes out below 80 °F, that is the heat index.
2. Otherwise use the regression by Lans P. Rothfusz, from an NWS Technical Attachment of 1990 (SR 90-23).
3. Apply an adjustment in very dry or very humid air, if one applies.

Worked examples

• 90 °F at 60% humidity. The simple formula gives 91.5 °F. That is 80 or more, so the regression takes over: 99.7 °F, about 100 °F. No adjustment applies. The NWS level is extreme caution.
• 104 °F at 8% humidity. The regression gives 98.0 °F. The air is drier than 13 percent, so 0.9 °F comes off: 97.2 °F. It feels about 7 degrees cooler than the air.
• 82 °F at 95% humidity. The regression gives 93.0 °F, and the humid-air adjustment adds 1.0 °F: 94.0 °F.
• 70 °F at 50% humidity. The simple formula gives 69.1 °F, below 80, so it stands. Mild air with moderate humidity feels about like the thermometer.
• In Celsius. 32 °C at 60% is 89.6 °F. The heat index is 98.7 °F, which is 37.1 °C. The temperature converter shows each conversion step.

Heat index table

Air temperature across, relative humidity down, heat index in °F, rounded to a whole degree. The NWS chart leaves cells above 137 °F empty and marks them extreme danger; they are marked off chart here.

Two things to notice. At 20 percent humidity, 100 °F feels like 97, so dry heat comes out close to the air temperature. At 60 percent, the same 100 °F feels like 129. And at 90 °F, from 40 percent up, every step of 10 points of humidity adds more than the step before it, because the regression multiplies temperature and humidity together.

NWS Amarillo gives the regression an error of ±1.3 °F, so a reading of 102 and one of 104 are not really different. The Weather Prediction Center also notes that the regression is not valid for extreme temperature and humidity beyond the range of Steadman's data.

Heat index danger levels

The NWS sorts heat index values into four levels by how likely heat disorders become with prolonged exposure or physical activity. The effects are quoted from the NWS Amarillo table.

One limit matters more than any other. The NWS says the heat index values were devised for shady, light wind conditions, and that exposure to full sunshine can increase them by up to 15 °F. Our 90 °F, 60% example sits at about 100 °F in the shade. In full sun it could feel like 115 °F, which is inside the danger range.

How the wind chill is calculated

The NWS defines the wind chill temperature as how cold people and animals feel outside, based on the rate of heat loss from exposed skin caused by wind and cold. The current formula took effect on November 1, 2001.

Where the 2001 formula came from

The index it replaced, used in the United States and Canada, went back to 1945 research by Antarctic explorers Siple and Passel. They measured how fast water cooled in a container hanging from a pole. Water in a container freezes faster than flesh, so the NWS says the old index underestimated the time to freezing and overestimated the chilling effect of the wind.

In the fall of 2000, the Office of the Federal Coordinator for Meteorological Services and Supporting Research organized a joint US and Canadian group, the Joint Action Group for Temperature Indices (JAG/TI), to build a new index. In early summer 2001, human trials ran at the Defence and Civil Institute of Environmental Medicine in Toronto. Six men and six women walked at 3 mph on a treadmill in a chilled wind tunnel, with sensors on their cheeks, forehead, nose and chin, in four trials of 90 minutes each. The results refined the formula and set the frostbite thresholds.

According to the NWS, the new index:

• estimates the wind at 5 feet, about the height of an adult face, from readings at the standard anemometer height of 33 feet,
• is based on a model of the human face and modern heat transfer theory,
• lowers the calm wind threshold from 4 mph to 3 mph,
• uses a consistent standard for skin tissue resistance,
• assumes no effect from the sun, as on a clear night.

Worked examples

• 0 °F with a 15 mph wind. V^0.16 is 1.5423. The formula gives −19.4 °F, so −19 °F. That matches the example the NWS uses, where exposed skin can freeze in 30 minutes.
• In Celsius. −10 °C is 14 °F, and 30 km/h is 18.6 mph (1 mph is exactly 1.609344 km/h). The wind chill is −3.1 °F, which is −19.5 °C. The wind chill calculator does these conversions for you.

Humidity is not in the formula. The NWS says relative humidity changed the result by less than 1 degree in its tests, so it was left out.

Wind chill table with frostbite times

Air temperature across, wind speed down, wind chill in °F. Where the NWS chart shades a cell, the time is how long exposed skin can take before frostbite develops: 30, 10 or 5 minutes. The zones follow the shading of the NWS chart cell by cell, as in the calculator.

Read across the 15 mph row. At 10 °F the wind chill is −7 °F with no frostbite zone. At 0 °F it is −19 °F and the 30 minute zone begins. At −20 °F it is −45 °F and the time drops to 10 minutes. Wind matters too: at −10 °F, going from 5 to 60 mph takes the wind chill from −22 to −48 °F.

What the chart does not tell you

• Frostbite needs freezing air. The NWS says you can only get frostbite if the actual air temperature is below freezing. A wind chill below 32 °F on a 35 °F day does not freeze skin, though hypothermia is still possible.
• Hypothermia is about body temperature. The NWS brochure defines it as an abnormally low body temperature, below 95 °F, with warning signs such as uncontrollable shivering, memory loss, disorientation, slurred speech and drowsiness. It says medical attention is needed immediately.
• Sun warms. Bright sunshine may raise the wind chill by 10 to 18 °F, according to the NWS.
• Only for people and animals. A car radiator or a water pipe cannot cool below the air temperature, however strong the wind.
• Tested on adults. The NWS advises being even more cautious with children, older people and anyone with compromised health.

When the wind chill gets dangerous, the NWS issues an advisory for wind chills that are potentially hazardous and a warning for wind chills that are life threatening.

Where the dew point fits

The NWS glossary calls the dew point a measure of atmospheric moisture: the temperature to which air must be cooled to reach saturation, with pressure and moisture held constant. A higher dew point means more moisture. Relative humidity, by contrast, compares the moisture with the most the air could hold at its current temperature, so it rises and falls with the temperature even when the moisture stays the same.

That is why the NWS office in La Crosse calls the dew point the better guide to how muggy it feels. Its example: at 30 °F with a 30 °F dew point the humidity is 100 percent, yet the air feels far less humid than at 80 °F with a 60 °F dew point and only 50 percent humidity. It gives these summer comfort bands:

Same moisture, different humidity

Hold the dew point fixed and let the day warm up. The relative humidity falls while the heat index climbs. Each cell shows the relative humidity and the heat index in °F, computed the way the heat index calculator does when you enter a dew point.

Look at the 65 °F row. The humidity drops from 71 to 37 percent between morning and afternoon, which sounds like the air got drier. It did not. The moisture is the same, and the heat index rose from 76 to 98 °F.

Comfort is not danger

The dew point bands describe comfort. The NWS heat danger levels belong to the heat index. Two cases show why the difference matters. A 95 °F afternoon at 25% humidity has a dew point of about 54 °F, in the dry and comfortable band, but a heat index of 93 °F, which is in the extreme caution level. A 75 °F night at 90% has an oppressive dew point of about 72 °F, yet a heat index of 76 °F, below the first level. Use the dew point calculator to find the dew point from temperature and humidity, and the heat index for the risk.

Which number to check

• Hot day, from about 80 °F: the heat index, against the four NWS levels. Add up to 15 °F if you will be in full sun.
• Cold day with wind, at or below 50 °F: the wind chill, and the frostbite time if the air is below freezing.
• Wondering how sticky it will feel: the dew point, in any season.
• Mild and calm: the thermometer is close enough. Neither index adds much there.

These indexes describe adults in the conditions the formulas assume, and the NWS advises extra caution with children, older people and anyone with compromised health. Follow the warnings from your local weather service, and get medical help right away for signs of heat stroke, frostbite or hypothermia.

Frequently asked questions

Sources

• NWS Weather Prediction Center, The Heat Index Equation
• NWS Amarillo, What is the heat index?
• NWS, Heat Index (safety page and chart)
• NWS, Wind Chill Chart, effective November 1, 2001 (PDF)
• NWS, Understanding Wind Chill
• NWS, Wind Chill Temperature Index brochure (PDF)
• NWS La Crosse, Dew Point vs Humidity
• NWS Glossary, Dew point

General information based on the sources above, not medical advice.

Written by Tobias. Updated October 20, 2026.