Dew Point Calculator - Humidity & Vapor Pressure
Free online dew point calculator and atmospheric hygrometry solver. Calculate dew point temperature, relative humidity, actual vapor pressure (hPa), and absolute humidity (g/m³) with human comfort level ratings using the high-precision Magnus-Tetens equation.
Dew Point Calculator & Vapor Pressure
Solve for Dew Point, Relative Humidity, or Temperature with Magnus-Tetens formula.
Input Values
Dew point of 60.2°F (15.6°C)
Dew Point Comfort Scale (°F / °C)
Dew Point & Vapor Pressure Mechanics: Magnus-Tetens Psychrometric Solver
The dew point is the temperature to which air must be cooled at constant pressure to achieve 100% relative humidity (saturation). Unlike relative humidity, which fluctuates wildly as temperatures change throughout the day, dew point provides an absolute measurement of atmospheric water vapor content.
The Magnus-Tetens Formula
The Magnus-Tetens approximation accurately models saturation vapor pressure over liquid water between -45°C and +60°C:
Dew Point (°C) = (b × α(T, RH)) / (a - α(T, RH))
Where a = 17.62, b = 243.12 °C, and α(T, RH) = (a × T) / (b + T) + ln(RH / 100).
- Below 50°F (10°C): Dry, crisp, and refreshing air.
- 50°F to 59°F (10°C - 15°C): Very comfortable for outdoor recreation.
- 60°F to 64°F (16°C - 18°C): Noticeably humid and sticky.
- 65°F to 69°F (18°C - 21°C): Muggy and uncomfortable for most people.
- 70°F+ (21°C+): Oppressive, tropical, and severe storm environment.
Step-by-Step Instructions
Enter Air Temperature
Input the ambient dry-bulb air temperature in degrees Celsius (°C) or Fahrenheit (°F).
Enter Relative Humidity (% RH)
Input current relative humidity percentage measured by your hygrometer or weather station.
Compute Dew Point & Vapor Pressure
The calculator applies the Magnus-Tetens approximation formula to compute Dew Point temperature and actual/saturation water vapor pressure in hectopascals (hPa).
Inspect Human Comfort Rating
Review the comfort classification (Dry, Comfortable, Humid, Muggy, Oppressive/Miserable) based on atmospheric dew point benchmarks.
Key Features & Capabilities
- High-precision Magnus-Tetens equation (`a = 17.27, b = 237.7°C`) yielding dew point accuracy within 0.1°C across -45°C to +60°C
- Calculates Actual Vapor Pressure (`E`) and Saturation Vapor Pressure (`E_s`) in hPa / millibars
- Absolute Humidity calculation outputting water vapor mass in grams per cubic meter (`g/m³`)
- Human comfort classification index based on Dew Point: Dry (<50°F), Comfortable (50-60°F), Sticky (60-65°F), Muggy (65-70°F), and Oppressive (>70°F)
- Multi-directional solving: calculate Relative Humidity from Temperature and Dew Point
- Dual unit support (°C and °F) with instant real-time computation
Frequently Asked Questions
What is Dew Point and what is the Magnus formula used to calculate it?
Dew Point is the temperature to which air must be cooled at constant barometric pressure for water vapor to condense into liquid water (dew). Using the Magnus-Tetens formula: `T_dp = (b * gamma) / (a - gamma)`, where `gamma = ((a * T) / (b + T)) + ln(RH / 100)`, with constants `a = 17.27` and `b = 237.7°C`.
Why is Dew Point a better measure of human comfort than Relative Humidity?
Relative humidity is relative to air temperature (50% RH at 95°F holds vastly more moisture than 50% RH at 50°F). Dew Point is an absolute measure of the moisture content in the air. A dew point above 65°F (18°C) feels sticky, and above 70°F (21°C) feels tropical and oppressive regardless of temperature.
What happens when Air Temperature and Dew Point are equal?
When the air temperature equals the dew point, the relative humidity is exactly 100%. The air is completely saturated with moisture, which leads to fog formation, heavy morning dew, low clouds, or condensation on cold surfaces.
How do you calculate Relative Humidity from Temperature and Dew Point?
Relative Humidity is calculated from the ratio of actual vapor pressure to saturation vapor pressure: `RH = 100 * (E(T_dp) / E_s(T)) = 100 * exp((a * T_dp)/(b + T_dp) - (a * T)/(b + T))`.
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