Relative Humidity Calculator

Calculate relative humidity, saturation vapor pressure, actual vapor pressure, and vapor pressure deficit based on air temperature and dew-point temperature. Relative Humidity Calculator helps meteorologists, engineers, researchers, and weather enthusiasts assess atmospheric moisture conditions.

Highlights

  • Calculates relative humidity from air and dew-point temperatures.
  • Provides saturation and actual vapor pressure measurements.
  • Determines vapor pressure deficit for environmental analysis.
  • Supports °C, °F, and K temperature units.
  • Displays pressure results in multiple units, including hPa, kPa, Pa, and psi.

Table of Contents

  1. How Relative Humidity Calculator Works?
  2. Practical Use Cases
  3. Accuracy and Limitations
  4. Quick Examples
  5. Frequently Asked Questions (FAQs)
  6. Cite This Calculator

How Relative Humidity Calculator Works?

The Relative Humidity Calculator estimates atmospheric moisture from the air temperature and dew-point temperature. It uses the Magnus formula to calculate saturation vapor pressure, which is the maximum water-vapor pressure that air can hold at a given temperature.

es=6.112⁢exp17.67⁢TT+243.5

Here, T is the air temperature and es is the saturation vapor pressure. The dew-point temperature is used in the same relationship to determine the actual vapor pressure in the air.

e=6.112⁢exp17.67⁢TdTd+243.5

The calculator then compares actual vapor pressure with saturation vapor pressure to obtain relative humidity. Vapor pressure deficit is the difference between the air's saturation vapor pressure and its actual vapor pressure.

RH=100⁢ees,VPD=es−e

OutputMeaningCalculation
Relative HumidityPercentage of the air's moisture-holding capacity currently filled100⁢e/es
Saturation Vapor PressureMaximum vapor pressure at the air temperaturees
Actual Vapor PressureWater-vapor pressure represented by the dew pointe
Vapor Pressure DeficitRemaining moisture capacity before saturationes−e

For example, with an air temperature of 20 °C and a dew point of 10 °C, the calculator determines the saturation vapor pressure at 20 °C, the actual vapor pressure associated with 10 °C, their ratio, and the resulting vapor pressure deficit.

Practical Use Cases

Meteorologists: Meteorologists can use relative humidity and vapor pressure deficit to describe atmospheric moisture, identify conditions favorable for fog or condensation, and support weather observations.

HVAC and building engineers: Building professionals can compare indoor or outdoor moisture conditions with condensation risks on surfaces, helping inform ventilation, humidity control, and environmental comfort assessments.

Agronomists and greenhouse growers: Plant specialists can use vapor pressure deficit to evaluate how strongly the air may draw moisture from leaves and to support irrigation and greenhouse climate decisions.

Environmental researchers: Researchers can calculate saturation and actual vapor pressure from temperature observations when studying evaporation, ecosystem moisture stress, and local atmospheric behavior.

Weather enthusiasts: Weather observers can enter air and dew-point temperatures to understand how close the atmosphere is to saturation and why muggy, dry, foggy, or condensation-prone conditions occur.

Accuracy and Limitations

The calculator uses the Magnus empirical vapor-pressure relationship, which generally provides good practical estimates for ordinary meteorological temperatures, especially around and above freezing. Its input controls allow values from -100 to 1000 for both temperatures, but results far outside the usual atmospheric range should not be treated as reliable because the Magnus approximation is not intended for extreme temperatures. The dew-point temperature should normally be no greater than the air temperature; otherwise, the calculated relative humidity can exceed 100% and the result may represent supersaturated or inconsistent observations. The calculation also assumes equilibrium water vapor and does not account for ambient pressure, dissolved substances, surface curvature, or separate ice-phase behavior below freezing.

Quick Examples

Example #1: Warm and Moderately Humid Air

Calculate atmospheric moisture conditions on a typical warm day using Celsius temperatures.

Example #2: Cold Winter Air

Assess relative humidity and vapor pressure in cold outdoor conditions using Fahrenheit temperatures.

Example #3: High-Altitude Atmospheric Conditions

Evaluate moisture levels in cold, low-temperature air using Kelvin temperature units.

Frequently Asked Questions

Relative humidity is the percentage of water vapor in the air compared with the maximum amount the air can hold at the same temperature. Higher values indicate air that is closer to saturation.

The dew point is the temperature at which air becomes saturated and condensation begins. A dew point closer to the air temperature generally indicates higher relative humidity.

Air is saturated when relative humidity reaches 100%. Further cooling or added moisture can cause condensation, fog, dew, or cloud formation.

Vapor pressure deficit is the difference between the moisture air can hold and the moisture it currently contains. It is commonly used to assess evaporation, plant water stress, and indoor environmental conditions.

Meteorologists, agricultural professionals, greenhouse operators, HVAC engineers, researchers, and weather enthusiasts use humidity measurements to evaluate atmospheric and environmental conditions.

Cite This Calculator