How to Calculate Humidity

How to Calculate Humidity

Humidity is defined as the quantity of water vapor that is present in the atmosphere. Humidity is measured in several forms such as absolute humidity and relative humidity.   This article on how to calculate humidity will help you in calculating both absolute and relative humidity. Relative humidity is the most important metric. It is used in forecasting weather reports. High humidity makes us feel hotter and increases our perspiration rate.
Calculating absolute humidity

• Absolute humidity is calculated on the basis of volume. It is the amount of water vapor in a cube of air. Formula used to calculate absolute humidity is the ratio of mass of water vapor divided by the mass of dry air in a volume of air at a given temperature.

Formula for calculating absolute humidity
In the above formula,
Mw = the molar mass of water vapor (18.02 kg/kmol)
Md = the molar mass of dry air (28.97 kg/kmol)
This is water vapor mass/ kilogram of dry air
Calculating relative humidity

• Relative humidity is the ratio of current absolute humidity to the highest possible absolute humidity that the air can hold.
• Relative humidity depends upon the air temperature. In other words, relative humidity is described as the amount of water vapor that a specific temperature can hold without water condensation.
• Relative humidity is the indicator of precipitation, fog and dew in the atmosphere, so it is mostly used in weather forecasting.
• Relative humidity does not change with the changes in air pressure. It remains constant with the changes in the air pressure as the actual vapor pressure and standard vapor pressure both change by the same factor.
• Relative humidity is expressed in percentage.

If you have the values for dewpoint and temperature and want to calculate the relative pressure then follow the steps below.

• Step 1: If you have the temperature and dewpoint values in Fahrenheit, then first convert them into degree Celsius as follows:

Tc=5.0/9.0* (Tf-32.0)

Tdc=5.0/9.0* (Tdf-32.0)
Where,
Tc and Tf are the air temperature in degree Celsius and Fahrenheit respectively
Tdc and Tdf are dewpoint temperature in degree Celsius and Fahrenheit respectively

• Step 2: Now, calculate the saturation vapor pressure (E_s) and actual vapor pressure (E) in millibars.

Es=6.11*10.0** (7.5*Tc/ (237.7+Tc))
E=6.11*10.0** (7.5*Tdc/ (237.7+Tdc))

• Step 3: Once you have calculated the saturation vapor pressure and the actual vapor pressure, you can compute the relative humidity by dividing the actual vapor pressure by saturation vapor pressure and multiplying it by 100 and express it in percentage.

Formula for this is
Relative Humidity in percent = RH = (E/Es)*100

• In the above formulae in step 2, 6.11 is the pressure coefficient for standard atmosphere. If you are applying  this formula above 7000 feet of altitude then you need to apply a pressure coefficient of 4.5 i.e. before applying this formula divide 6.11 by 4.5. By doing this you will get the value 1.38. Use this value in both the formulae.
• If you already have the temperature values in degree Celsius then skip the first step of converting the temperature and dewpoint values from Fahrenheit to degree Celsius.
• There are a large number on calculators online to calculate the humidity. They calculate the humidity depending upon your location and use a large number of advanced technologies in calculations.