Weather Calculators
Free online weather calculators for wind chill, heat index, dew point, humidity, rainfall volume, and snow day probability. These meteorology tools use official formulas from the National Weather Service and scientific literature to calculate feels-like temperatures, moisture levels, and precipitation volumes. All tools work in your browser with no sign-up required.
Wind Chill Calculator
Calculate wind chill temperature using the NWS formula. Shows frostbite risk time and danger level based on air temperature and wind speed.
Heat Index Calculator
Calculate heat index (feels-like temperature) using the Rothfusz regression equation. Shows danger level and health advisory based on temperature and humidity.
Dew Point Calculator
Calculate dew point temperature from air temperature and relative humidity using the Magnus formula. Shows comfort level and fog likelihood.
Humidity Calculator
Calculate relative humidity, dew point, or wet bulb temperature. Three calculation modes with comfort assessment and condensation risk.
Rainfall Calculator
Calculate total water volume from rainfall over an area in gallons, liters, and cubic feet. Convert between rainfall rate units.
Snow Day Calculator
Estimate snow day probability based on snowfall amount, temperature, wind speed, school type, and geographic region.
How to Use Weather Calculators
Weather calculators help you understand how atmospheric conditions affect comfort and safety. Use the Wind Chill Calculator when temperatures drop below 50°F with wind present to determine frostbite risk. Use the Heat Index Calculator in summer when humidity makes temperatures feel hotter than the actual reading. The Dew Point Calculator helps predict fog, condensation, and comfort levels. Enter the current temperature and relevant conditions (wind speed or humidity), and get instant results with safety advisories.
Weather Calculation Formulas
Wind Chill (NWS): WC = 35.74 + 0.6215T - 35.75V^0.16 + 0.4275TV^0.16
where T = air temp (°F), V = wind speed (mph)
Heat Index (Rothfusz): HI = -42.379 + 2.04901523T + 10.14333127R
- 0.22475541TR - 0.00683783T^2 - 0.05481717R^2
+ 0.00122874T^2R + 0.00085282TR^2 - 0.00000199T^2R^2
Dew Point (Magnus): Td = (b x alpha) / (a - alpha)
where alpha = (aT)/(b+T) + ln(RH/100)
a = 17.27, b = 237.7°C
Rainfall Volume: V = Area x Depth
The NWS wind chill formula was updated in 2001 based on advances in science, technology, and computer modeling. The Rothfusz heat index regression is the standard used by the National Weather Service for heat advisories. The Magnus formula provides an accurate approximation of dew point from temperature and relative humidity.
Example Calculation
Wind Chill Example:
Air Temperature (T) = 20°F
Wind Speed (V) = 15 mph
WC = 35.74 + 0.6215(20) - 35.75(15^0.16) + 0.4275(20)(15^0.16)
WC = 35.74 + 12.43 - 35.75(1.534) + 0.4275(20)(1.534)
WC = 35.74 + 12.43 - 54.84 + 13.12
WC = 6.4°F
Feels like 6°F — Frostbite risk in 30 minutes on exposed skin.
Wind Chill Reference Table
| Air Temp (°F) | 10 mph | 20 mph | 30 mph | 40 mph |
|---|---|---|---|---|
| 40 | 34 | 30 | 28 | 27 |
| 30 | 21 | 17 | 15 | 13 |
| 20 | 9 | 4 | 1 | -1 |
| 10 | -4 | -9 | -12 | -15 |
| 0 | -16 | -22 | -26 | -29 |
| -10 | -28 | -35 | -39 | -43 |
| -20 | -41 | -48 | -53 | -57 |
Frequently Asked Questions
What is wind chill and when does it apply?
Wind chill is the perceived decrease in air temperature felt by the body due to wind. It only applies when the air temperature is at or below 50°F and wind speed is above 3 mph. Wind chill does not affect objects like cars or pipes — it only describes how cold it feels on exposed human skin.
What is the heat index and why is it dangerous?
The heat index combines air temperature and relative humidity to determine how hot it actually feels. High humidity prevents sweat from evaporating efficiently, reducing the body's ability to cool itself. A heat index above 103°F is considered dangerous, and above 125°F is extremely dangerous with high risk of heat stroke.
What does dew point tell you about comfort?
Dew point is the temperature at which air becomes saturated and water vapor condenses. A dew point below 55°F feels comfortable, 55-65°F feels slightly humid, and above 65°F feels oppressive. Unlike relative humidity, dew point gives a consistent measure of moisture regardless of temperature.
How is rainfall volume calculated?
Rainfall volume equals the catchment area multiplied by the depth of rain. One inch of rain on 1,000 square feet produces about 623 gallons of water. This calculation is useful for rainwater harvesting, drainage planning, and understanding flood risk in your area.
How accurate is the snow day calculator?
The snow day calculator is an educational estimation tool, not a weather forecast. It considers factors like snowfall amount, temperature, wind speed, school type, and region to produce a probability score. Actual school closure decisions depend on road conditions, timing, and local policies that vary by district.