Relative Humidity Calculator

Created by Purnima Singh, PhD
Reviewed by Steven Wooding
Based on research by
Mark G. Lawrence The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications Bulletin of the American Meteorological Society 86(2) (01 Feb 2005)
Last updated: Jun 07, 2022

The relative humidity calculator allows you to determine the relative humidity from the air temperature and dew point.

Remember those hot and humid months during summer and how it feels? The warm and muggy feeling can be attributed to high relative humidity that limits the amount of sweat evaporated from our skin. Less sweat evaporation means we feel much hotter than the actual temperature. To know more about relative humidity and how it affects us, continue reading this article. You will find answers to questions like:

  • What is relative humidity?
  • How to calculate relative humidity.
  • How to measure relative humidity.

We have also included an example of calculating relative humidity using our calculator.

What is relative humidity? – Relative humidity definition

The relative humidity is a measure of how moist the air is. We can define relative humidity as the actual amount of water vapor (moisture) present in the air compared to the total amount of water vapor that the air can hold at a given temperature.

To express relative humidity as the ratio of vapor pressure PwP_w to the saturation vapor pressure PwsP_{ws} at a given temperature, we can use the equation:

RH=100×PwPws RH = 100 \times \frac{P_w}{P_{ws}}

Note that we always express relative humidity as a percentage.

Another concept that we should be familiar with while talking about relative humidity is absolute humidity. It is the mass of water vapor present in a unit volume of air. We can write the formula for absolute humidity as:

H=mV H = \frac{m}{V}

where:

  • HH – Absolute humidity usually expressed in g/m3;
  • mm – Mass of water vapor; and
  • VV – Volume of air and water vapor mixture.

How to calculate relative humidity

We can express the relation between dew point, air temperature, and relative humidity as:

Dp=λ×(ln(RH100)+β Tλ+T)β(ln(RH100)+β Tλ+T) D_p = \frac{\lambda \times \bigg (\ln \big(\frac{RH}{100} \big ) + \frac{\beta\ T}{\lambda + T} \bigg)}{\beta - \bigg (\ln \big(\frac{RH}{100} \big ) + \frac{\beta\ T}{\lambda + T} \bigg)}

where:

  • DpD_pDew point temperature (in °C);
  • RHRH – Relative humidity;
  • TT – Air temperature; and
  • β=17.625\beta = 17.625 and λ=243.04°C\lambda = 243.04 \text{\degree C} – Revised Magnus coefficients as recommended by Alduchov and Eskridge.

Substituting the values and rearranging the above equation, we can write the formula for relative humidity as:

RH=100×[e17.625×Dp243.04+Dpe17.625×T243.04+T] RH = 100 \times \left [ \frac{e^{\frac{17.625 \times D_p}{243.04 + D_p}}}{e^{\frac{17.625 \times T}{243.04 + T}}} \right ]

If you want to learn how to calculate relative humidity from the actual and saturation mixing ratios of air, we recommend checking our mixing ratio of air calculator.

How to use the relative humidity calculator

Now let's see how we can use the relative humidity (RH) calculator to find out the relative humidity when the dew point is 70 °F and the air temperature is 95 °F.

  1. Enter the air temperature as 95 °F (35 °C). The tool has an inbuilt temperature conversion option that allows you to input the temperature in other units.

  2. Type the dew point temperature as 70 °F (~21.1 °C).

  3. The RH calculator will display the relative humidity as 44.48%.

  4. You can also use this tool to calculate air temperature and dew point.

How do we measure relative humidity?

To measure relative humidity, we use hygrometers. Some commonly used hygrometers are:

  1. Psychrometer: The psychrometers measure the relative humidity by measuring the difference between dry bulb temperature and wet bulb temperature.
  2. Hair hygrometer: This device relies on measuring the relationship between the relative humidity and length of hair.
  3. Capacitive type electronic hygrometer: These hygrometers detect the change in a sensor's capacitance when it absorbs moisture and thus measures the humidity.
  4. Chilled-mirror dew point hygrometer: As the name suggests, these hygrometers measure the dew point temperature, which we can then use to calculate relative humidity.

What is an ideal level of relative humidity?

In general, we feel more comfortable when the relative humidity level is between 30-50%. Lower humidity levels can lead to dryness, irritation, and itching.

Levels higher than 50% are uncomfortable as our sweat won't evaporate efficiently and we will "feel" a higher temperature. Higher humidity levels also cause mold growth which can lead to various health issues.

FAQ

How do I calculate relative humidity with temperature and dew point

To calculate relative humidity with temperature and dew point, follow the given instructions:

  1. Measure the air temperature T, in °C.

  2. Find out the dew point temperature Dp, in °C.

  3. Calculate relative humidity RH using the formula, RH = 100 × {exp[17.625 × Dp/(243.04 + Dp)]/exp[17.625 × T/(243.04 + T)]}.

What does it mean when relative humidity is 100?

When relative humidity is 100%, it means that air is fully saturated with water. Relative humidity measures the saturation level of air with water; hence a relative humidity of 100% implies that the air cannot take any more moisture.

What happens to relative humidity when temperature increases?

If a system's moisture content remains constant (e.g., in a closed room), the relative humidity decreases with an increase in temperature. This outcome is because the higher the temperature, the higher the amount of water vapor the air can hold, and hence the lower the relative humidity.

Purnima Singh, PhD
Temperature
°F
Dew point
°F
Relative humidity
%
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