How To Calculate Relative Humidity: From Different Entities And Facts


Relative Humidity(RH) measurement plays a crucial role in different fields. How to calculate Relative Humidity” is the main concern of this article.

To calculate Relative Humidity we use different devices and methods. Many applications like air conditioning systems, food storage, weather forecasting, wood drying process, cooling tower, etc. require regular monitoring of relative humidity.

In simple words, relative humidity is the ratio of the current amount of water vapor in the air to the maximum capacity of holding water vapor by the air at a particular temperature.

Relative Humidity has changed along with the change in temperature.

The Relative Humidity can be expressed as follows:

Relative Humidity(%)=[latex] \frac{Actual Vapor Density}{Saturation Vapor Density}X100%[/latex]

The relative humidity is always expressed in percentage(%).

If the Relative Humidity is 50% that means the air holds a half of the water vapor it can theoretically hold.

Generally, a psychrometer or hygrometer containing both wet-bulb and dry-bulb thermometers is used as RH measuring device.

Relative Humidity Measuring Device; Image Credit: Pixabay

How to calculate Relative humidity with Wet Bulb and Dry Bulb?

Relative humidity can be calculated from the difference between the temperature on the wet-bulb thermometer and on the dry-bulb thermometer and using a relative humidity chart.

Using two thermometers (dry bulb and wet bulb) we can determine the Relative Humidity at our home or an area and can know if too much or too little moisture in it. The Relative humidity percentage has a great difference at various temperatures because warmer air holds more moisture than cooler air.

Wet Bulb Thermometer; Image Credit: Wikipedia

The steps followed to calculate the Relative Humidity with Wet Bulb and Dry Bulb thermometer as follows:

Take two bulb thermometers side by side and wrap a water soaked cloth on one of the two thermometers.

After some time, observe the temperature reading of both the thermometers and note down the readings in degrees Fahrenheit or degrees Celsius.

The dry thermometer gives air temperature which is true thermodynamic temperature. The temperature reading obtained from the wet-bulb is the lowest one due to evaporation from the wetted surface.

Since the wet-bulb temperature reading is less than the dry-bulb temperature, we will subtract  [latex]T_{wb}[/latex] from [latex]T_{db}[/latex] to get the difference.

Now using the psychrometric chart we can calculate the RH.

how to calculate relative humidity
Psychrometric Chart; Image Credit: Wikipedia

How to Calculate Humidity in a Room?

We know the Relative Humidity in a room, we can use a device called hygrometer. Over the centuries these instruments has changed, advanced and different types as per the user’s requirement.

The main use of hygrometer is to measure the amount of water vapor in air, apart from that it has numerous commercial uses. Now a days as per the requirement or applications different varieties of hygrometers are available in the market.

An excessive amount of moisture content in the air creates molds and pollutants inside the houses while not an adequate amount results in skin irritation, breathing-related problems, electrostatic discharge, etc. To maintain a proper humidity level we can use dehumidifiers or vaporizers to adjust the environmental humidity manually.

Hygrometer; Image Credit: Wikipedia

How to Calculate Relative Humidity with Temperature?

Relative Humidity indicates the percentage of moisture present in comparison to the maximum amount of moisture in the air at a specific temperature.

The Relative Humidity is higher at a lower temperature because cold air can hold more water vapors than warm air. The RH is temperature specific and tends to change with temperature changes too.

Relative Humidity is inversely proportional to the temperature for a fixed amount of water vapor.

The Relative Humidity can be expressed as follows:

Relative Humidity(%)=[latex]\frac{Actual Vapor Pressure}{Saturation Vapor Pressure}X100%[/latex] Eq(1)

As the denominator value in equation(1) increases, the RH value obtained is decreased.

Therefore when saturated water vapor pressure decreases (with temperature decrement), related humidity value increases until air gets saturated.

How to Calculate Relative Humidity from Humidity?

Humidity refers to the amount of water vapor present in the air.

In general, we can say humidity is measured in three different ways: Relative, Absolute, and Specific Humidity which indicate moisture content in the air in three different ways.

The Relative Humidity can be expressed as follows:

Relative Humidity(%)= [latex]\frac{Actual Vapor Density}{Saturation Vapor Density}X100%[/latex]

How to Calculate Relative Humidity using a Sling Psychrometer?

Sling Psychrometers are a type of battery-free hygrometer, most widely used for certain characteristics like inexpensive, easy to use, low maintenance, portable, etc.

Steps followed to measure the Relative Humidity are as follows:

  • Take off the reservoir cap of the psychrometer and fill the reservoir with water.
  • Fill the reservoir with water to make the wick saturated, tightly secure the lid of the chamber. Similarly, make sure that the wick at the end portion of the thermometer must be wet.
  • Keep the mercury reservoir in the dry-bulb thermometer dry.
  • Now in the area where we have to measure the Relative Humidity, swing the psychrometer speedily in the air for a minute or two.
  • Stop whirling the psychrometer to stabilize the temperatures, note down the readings from both the thermometers.
  • Now calculate the wet-bulb depression value by subtracting wet bulb temperature from dry bulb temperature.
  • With the help of the attached table or psychromatic chart determine RH value. For that locate the dry bulb temperature on the horizontal axis of the chart, and then mark the wet bulb depression value on the vertical axis.
  • Now mark the intersection point of the two readings to obtain the Relative Humidity value in percentage.
  • For more accuracy repeat the method for two /three times.
Sling Psychrometers; Image Credit: Flickr

A large difference between the dry and wet bulb temperature indicates low RH value and on the contrary a small difference is indicating towards higher RH value. If the dry and wet bulb temperatures are equal then we can say Relative Humidity is 100%.

How to calculate Relative Humidity with Temperature and Vapor pressure?

Saturated vapor pressure increases with increasing temperature, therefore Relative Humidity decreases with increasing temperature.

The Relative Humidity can be expressed as follows:

Relative Humidity(%)=[latex] \frac{Actual Vapor pressure}{Saturation Vapor pressure}X100%[/latex]

How to Calculate Relative Humidity with Temperature and Dew Point?

Dew point is the temperature at which air must be cooled for saturation to occur (dew to form)and to reach100% Relative Humidity.

The steps followed to calculate RH using known temperature and dew point are described below:

  • The temperature of air and dew point must be in Celsius, if not then convert to degree Celsius.
  • We can convert Fahrenheit into Celsius using the formula

               [latex]C=\frac{5}{9}(F-32)[/latex]   Eq(1)

               where C= Celsius temperature, F= Fahrenheit temperature,

  • Now calculate the Saturated Vapor Pressure[latex](E_{s})[/latex] using the formula

                [latex]E_{s}=6.11\cdot\exp{\frac{7.5T}{273.3+T}}[/latex] Eq(2)

                where T=Air Temperature, [latex]E_{s}[/latex]=Standard Vapor Pressure

  • Now determine the Actual Vapor Pressure(E) using the same formula i.e Eq(2)

                [latex]E=6.11\cdot\exp{\frac{7.5T_{d}}{273.3+T_{d}}}[/latex]

                Where [latex]T_{d}[/latex]=Dew Point, E= Actual Vapor Pressure

  • Now calculate the Relative Humidity(RH) by dividing Actual Vapor Pressure by Saturated Vapor Pressure and multiplying by 100.

[latex]RH=\frac{E}{E_{s}}X100[/latex]    Eq(3)

The sling psychrometer can also be used to measure dew point. Dew point is simply the relative humidity with 100 per cent or complete saturated state, condensation starts at that point. Dew Point around 55 is preferable for human beings.

Sangeeta Das

I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at https://www.linkedin.com/in/sangeeta-das-57233a203/

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