Weather Calculators: Wind Chill, Heat Index, and Dew Point Explained

The thermometer says 35 degrees Fahrenheit. The wind is blowing at 20 mph. You step outside and it feels like 24 degrees. Why? Because your body loses heat faster when wind carries away the warm air layer next to your skin. That is wind chill, and it has a precise mathematical formula.

On the other end, the thermometer says 92 degrees with 65% humidity. It feels like 105 degrees. That is heat index, and it can be the difference between an uncomfortable afternoon and a medical emergency.

Weather calculators take raw atmospheric data (temperature, wind speed, humidity) and translate it into what it actually feels like to a human body. The National Weather Service (NWS) uses three tools to assess hazardous heat in 2026: Heat Index, Wet Bulb Globe Temperature, and experimental HeatRisk. This guide covers the formulas behind wind chill, heat index, and dew point so you can calculate them yourself.

Wind Chill: How Cold It Really Feels

Wind chill is the temperature your skin feels when wind speed and air temperature are combined. The NWS uses a formula developed in 2001 based on heat transfer from human skin.

The NWS Wind Chill Formula

Wind Chill (degrees F) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)

Where:

• T = air temperature in degrees Fahrenheit
• V = wind speed in mph
• ^ = raised to a power

Valid Range

Wind chill is only defined for:

• Temperatures at or below 50 degrees F
• Wind speeds above 3 mph

Above 50 degrees or below 3 mph wind, wind chill is not calculated. The formula also assumes no impact from sun. Bright sunshine can increase the wind chill temperature by 10 to 18 degrees F.

Example Calculation

Temperature: 20 degrees F. Wind: 15 mph.

WC = 35.74 + (0.6215 x 20) - 35.75 x (15^0.16) + 0.4275 x 20 x (15^0.16) WC = 35.74 + 12.43 - 35.75 x 1.444 + 8.55 x 1.444 WC = 35.74 + 12.43 - 51.62 + 12.35 WC = 8.9 degrees F

So 20 degrees with 15 mph wind feels like 9 degrees. That is a 11-degree difference. Exposed skin can develop frostbite in 30 minutes at this wind chill.

Wind Chill Chart (Selected Values)

Temp (F)	5 mph	10 mph	15 mph	20 mph	25 mph	30 mph
40	36	34	32	30	29	28
30	25	21	19	17	16	15
20	13	9	6	4	3	1
10	1	-4	-7	-9	-11	-12
0	-11	-16	-19	-22	-24	-26
-10	-22	-28	-32	-35	-36	-38

Source: NWS Wind Chill Temperature Index, 2026.

Frostbite Risk by Wind Chill

Wind Chill	Frostbite Time
0 to -15 F	30 minutes
-16 to -27 F	15 to 30 minutes
-28 to -39 F	10 to 15 minutes
-40 to -47 F	5 to 10 minutes
-48 F or below	Under 5 minutes

Heat Index: How Hot It Really Feels

Heat index is what the temperature feels like when relative humidity is combined with air temperature. The NWS developed the formula based on human skin cooling through perspiration.

The NWS Heat Index Formula

The full Rothfusz regression equation:

HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541T(RH) - 0.00683783T^2 - 0.05481717RH^2 + 0.00122874T^2(RH) + 0.00085282T(RH^2) - 0.00000199T^2(RH^2)

Where T is temperature in degrees F and RH is relative humidity in percent.

Adjustments

• If RH is below 13% and temp is 80 to 112 F: subtract an adjustment for dry air
• If RH is above 85% and temp is 80 to 87 F: add an adjustment for very humid conditions

Example Calculation

Temperature: 90 degrees F. Humidity: 60%.

HI = -42.379 + (2.049 x 90) + (10.143 x 60) - (0.225 x 90 x 60) - (0.007 x 90^2) - (0.055 x 60^2) + (0.001 x 90^2 x 60) + (0.001 x 90 x 60^2) - (0.000002 x 90^2 x 60^2)

HI = -42.379 + 184.41 + 608.58 - 1215 - 56.7 - 197.28 + 486 + 194.4 - 5.832

HI = approximately 96.2 degrees F

So 90 degrees with 60% humidity feels like 96 degrees. The body cannot cool itself as efficiently when humidity is high because sweat evaporates more slowly.

Heat Index Chart (Selected Values)

Temp (F)	40% RH	50% RH	60% RH	70% RH	80% RH
80	80	81	82	83	85
85	86	88	90	93	97
90	91	95	100	106	113
95	97	103	110	119	130
100	103	111	121	132	144

Source: NWS Heat Index Chart, 2026.

Heat Safety Categories

Heat Index	Category	Health Risk
80 to 90 F	Caution	Fatigue possible with prolonged activity
90 to 103 F	Extreme Caution	Heat cramps, heat exhaustion possible
103 to 125 F	Danger	Heat cramps, heat exhaustion likely, heat stroke possible
125 F+	Extreme Danger	Heat stroke highly likely

The NWS notes that full sunshine can increase heat index values by up to 15 degrees F. The heat index is calculated for shady conditions with light wind.

Dew Point: The Comfort Indicator

Dew point is the temperature at which air becomes saturated and water begins to condense. It is a more accurate measure of comfort than relative humidity because it directly measures how much moisture is in the air.

Calculating Dew Point

The Magnus-Tetens approximation:

Dew Point (Tdp) = (b x alpha(T,RH)) / (a - alpha(T,RH))

Where:

• alpha = (a x T) / (b + T) + ln(RH/100)
• a = 17.625
• b = 243.04 degrees C
• T = temperature in degrees C
• RH = relative humidity in percent

Dew Point Comfort Scale

Dew Point (F)	Comfort Level
Below 50	Very dry, comfortable
50 to 55	Dry, comfortable
55 to 60	Comfortable
60 to 65	Starting to feel humid
65 to 70	Humid, somewhat uncomfortable
70 to 75	Very humid, uncomfortable
75+	Extremely humid, oppressive

A dew point of 72 degrees means the air feels sticky and oppressive regardless of the actual temperature. A dew point of 45 degrees means the air is dry and comfortable even at 90 degrees.

Why Dew Point Is Better Than Relative Humidity

Relative humidity changes with temperature even when the amount of moisture in the air stays the same. A morning at 70 degrees with 90% humidity has the same absolute moisture as an afternoon at 90 degrees with 50% humidity. But the dew point stays constant at approximately 67 degrees in both cases. Dew point tells you how much moisture is actually in the air.

Temperature Conversion

The three temperature scales used worldwide:

Scale	Freezing Point of Water	Boiling Point of Water
Fahrenheit (F)	32	212
Celsius (C)	0	100
Kelvin (K)	273.15	373.15

Conversion Formulas

• F to C: C = (F - 32) x 5/9
• C to F: F = (C x 9/5) + 32
• C to K: K = C + 273.15
• F to K: K = (F - 32) x 5/9 + 273.15

Quick Estimation

For a rough F-to-C conversion: subtract 30 and divide by 2. For 80 degrees F: (80 - 30) / 2 = 25 degrees C. The exact answer is 26.7 degrees C. Close enough for everyday use.

Use our Temperature Converter to convert between Fahrenheit, Celsius, and Kelvin instantly.

Real-World Scenarios

Scenario 1: Winter Running Decision

It is 25 degrees F with 20 mph wind. Should you run outside?

Wind chill: 35.74 + (0.6215 x 25) - 35.75 x (20^0.16) + 0.4275 x 25 x (20^0.16) = 35.74 + 15.54 - 35.75 x 1.448 + 10.69 x 1.448 = 35.74 + 15.54 - 51.77 + 15.48 = 14.99 degrees F.

Wind chill of 15 degrees is in the "caution" zone. Frostbite time is approximately 30 minutes for exposed skin. You can run if you cover all exposed skin (hat, gloves, face mask) and limit your run to under 30 minutes.

Scenario 2: Outdoor Event Safety

You are planning an outdoor event. The forecast is 93 degrees F with 70% humidity.

Heat index: Using the NWS chart, 93 degrees at 70% humidity gives a heat index of approximately 113 degrees F. This falls in the "Danger" category. Heat cramps and heat exhaustion are likely with prolonged activity. Consider moving the event indoors, providing shade and water stations, or scheduling for early morning when temperatures are lower.

Scenario 3: Home Humidity Management

Your indoor temperature is 72 degrees F with 65% relative humidity. What is the dew point?

Convert to Celsius: C = (72 - 32) x 5/9 = 22.2 degrees C. alpha = (17.625 x 22.2) / (243.04 + 22.2) + ln(65/100) = 391.88 / 265.24 + (-0.431) = 1.477 - 0.431 = 1.046 Dew point = (243.04 x 1.046) / (17.625 - 1.046) = 254.22 / 16.579 = 15.33 degrees C = 59.6 degrees F.

A dew point of 60 degrees is at the upper end of "comfortable." You might consider running a dehumidifier to bring the dew point below 55 degrees for optimal comfort.

Common Mistakes

1. Using wind chill above 50 degrees. Wind chill is only defined for temperatures at or below 50 degrees F. Above 50, wind has a cooling effect but the NWS formula does not apply.

2. Using heat index below 80 degrees. The Rothfusz regression is not accurate below approximately 80 degrees F. Below 80, a simpler formula is used.

3. Confusing dew point with relative humidity. Relative humidity changes with temperature. Dew point stays constant unless moisture is added or removed. Dew point is the better comfort indicator.

4. Forgetting sun exposure. Wind chill assumes clear night sky (no sun). Heat index assumes shade. Full sun can increase heat index by up to 15 degrees and wind chill by 10 to 18 degrees.

5. Ignoring the wet bulb globe temperature. For active individuals (athletes, construction workers), WBGT is a better heat stress indicator than heat index because it accounts for sun exposure, wind, and radiant heat. The NWS added WBGT to its heat tools in 2026.

External Research and Resources

• National Weather Service: Wind Chill FAQ provides the official NWS wind chill formula, calculation method, and frostbite timing guidance.
• NOAA Weather Prediction Center: Heat Index Equation documents the complete Rothfusz regression equation with adjustment formulas for extreme humidity conditions.
• NWS Heat Tools 2026 (PDF) describes the three NWS heat assessment tools: Heat Index, Wet Bulb Globe Temperature, and experimental HeatRisk.

People Also Ask

What is the wind chill formula?

The NWS wind chill formula is: WC = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16), where T is temperature in Fahrenheit and V is wind speed in mph. Wind chill is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Use our Wind Chill Calculator for instant results.

What is the heat index and how is it calculated?

Heat index is what the temperature feels like when humidity is combined with air temperature. The NWS uses the Rothfusz regression equation, a multi-variable formula involving temperature and relative humidity. At 90 degrees F and 60% humidity, the heat index is approximately 96 degrees. Use our Heat Index Calculator for instant results.

What is dew point and why does it matter?

Dew point is the temperature at which air becomes saturated and water condenses. It is a direct measure of moisture in the air. A dew point below 55 degrees feels dry and comfortable. A dew point above 70 degrees feels oppressive. Dew point is a better comfort indicator than relative humidity because it does not change with temperature.

How do I convert Fahrenheit to Celsius?

C = (F - 32) x 5/9. For example, 80 degrees F = (80 - 32) x 5/9 = 26.7 degrees C. To convert Celsius to Fahrenheit: F = (C x 9/5) + 32. Use our Temperature Converter for instant conversions between Fahrenheit, Celsius, and Kelvin.

Related Calculators

• Wind Chill Calculator - Calculate wind chill from temperature and wind speed
• Heat Index Calculator - Calculate heat index from temperature and humidity
• Dew Point Calculator - Calculate dew point from temperature and humidity
• Temperature Converter - Convert between Fahrenheit, Celsius, and Kelvin
• Unit Converter - Convert between any units of measurement