How To Control Relative Humidity: Increase, Decrease In Different Circumstances

This article discusses about how to control relative humidity. Humidity is the presence of water droplets in air. Humid air is moist in nature. In this article we shall study about relative humidity.

Relative humidity is an important term used in refrigeration and air conditioning applications. It is also used by the meterological department to get the actual facts about how humid the weather is. In this article we shall study about relative humidity, specific humidity and how to control relative humidity.

What is relative humidity?

Relative humidity is the measure of how much water vapour is present in an air-water mixture compared to the maximum amount of water tha can be present inside the mixture.

The relative humidity is simply a ratio of amount water present in the water air mixture and maximum amount of water that can be added to the mixture. When this ratio is equal to one that is both the numerator and denominator are same, then the mixture is said to be saturated as no more water can be handled by the mixture.

Relative humidity formula

We have defined relative humidity in above section. Now let us have a look on the formula of relative humidity.

It is given below-

φ = Pv/Ps


Phi is relative humidity

Pv is partial pressure of vapour

Ps is the saturation pressure

What is specific humidity?

Specific humidity is the measure of amount of water vapour present in a unit mass of water air mixture. It can be defined as the ratio of mass of water present in the solution to the mass of solution.

Mathematically, specific humidity can be given as-

ω = mv/ma


omega is the specific humidity

mv is the mass of vapour present in the vapour

ma is the mass of air water mixture

how to control relative humidity
Image: Mollier Diagram

Image Credits: Markus SchweissHS-Wasserdampf englCC BY-SA 3.0

How to increase relative humidity?

As we know, relative humidity is the ratio of amount of water in the mixture to the maximum amount of water that can be added in the air water mixture. The denominator term is constant. So to increase the ratio, we need to increase the numerator that is increasing the water content in the mixture.

This can be done by many ways. Some of the methods are given below-

  • Using a vaporiser or room humidifier.
  • Venting the dryer indoors.
  • Drying clothes inside the room.
  • Boiling water on stove.
  • Cooking at home.
  • Keeping houseplants.

How to reduce relative humidity?

As already discussed in above section, to increase or decrease the relative humidity we need to vary the numerator term that is the amount of water in the mixture.

To reduce relative humidity of room we may take help of following methods-

  • Using air conditioner
  • Using an air dehumidifier.
  • Taking cooler showers
  • Drying clothes outside.
  • Actively using exhaust fans/ventilation fans
  • Fixing leaking pipes if any.

How to control relative humidity in a room?

We have already discussed about increasing and decreasing the relative humidity in rooms. We can simply do this by humidifying de humidifying the room.

We can do this by using the methods discussed above. It is important to regulate the relative humidity inside a room because more humidity can cause discomfort due to excessive sweating. And less humidity can leave your skin dry causing discomfort again. Hence an optimum value of relative humidity is desirable.

How to control relative humidity in clean rooms?

Clean rooms are those rooms where the impurities levels are way below than normal rooms we find in our houses and classrooms. Such rooms are used in assembly of rocket vehicles, pharamceuticals etc where any impurity can cause a fatal damage to the system.

Commonly two approaches are used to control the humidity, they are air conditioning and dessicants. First the air is cooled below the dew point of that airstream. This way excess water vapour condenses which dehumidifies the air stream. The air is again reheated to desired temperature and then sent back to the room.

How to control relative humidity in hvac

There are four common ways to enhance relative humidity in hvac. These strategies are given below-

  • The most important and cheapest way is to clean the coils. Any impurities will decrease the efficiency.
  • A preventative maintenance plan can also reduce the chances of damage.
  • Adjusting the air flow can also play an important role in regulating the relative humidity.
  • Installing a de humidifier in the system.

How to control relative humidity in greenhouse?

Greenhouse is a structure with walls and transparent roof. This structure is made to grow plants which require a certain set of temperature and humidity conditions to grow.

To reduce humidity in a greenhouse, we should give adequate water to plants, the plant spacing should also be adequate. The floor should be well drained, the air should be moving that means the place should be well ventilated.

How to control relative humidity in ware house?

Warehouses are simply storage places for keep the inventory. It is very important to control the relative humidity at such places, this can be done by following methods-

  • Using of complementary dehumidification systems.
  • Bringing the outside air in the room.
  • Installing HVAC systems.
  • Using of fans inside the rooms.

How to control high relative humidity?

To control high relative humidity we use following methods which reduce the relative humidity and bring it down to normal levels-

  • Air conditioners bring down relative humidity as per desired levels.
  • Using exhaust fans or ventilation fans.
  • Taking cooler showers
  • Moving house plants from one place to another
  • Drying the laundry outside of the room

When is the relative humidity lowest?

The relative humidity is not constant throughout the day. It changes with the amount of heat striking the surface of Earth.

Relative humidity is lowest after mid day. The relative humidity keeps decreasing after night and dawn. The lowest being during the mid day or afternoon. This is a general case. The relative humidity value is different in coastal areas due to the prsence of dew.

When is the relative humidity highest?

As discussed in the earlier section, the relative humidity changes with the movement of sun as theamount of heat striking on Earth keeps changing with varying sunlight.

The relative humidity is maximum during the night time. The value of relative humidity keeps decreasing after passing midnight. The relative humidity is different at different places for example at coastal areas the relative humidity’s value will be different and places away from coastal areas will have different values of relative humidity.


Hi ....I am Abhishek Khambhata, have pursued B. Tech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth-year project was based on the performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like-minded people.

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