Relative Humidity And Temperature: Relationship, Graph, Formula, Chart


This article discusses the relationship between relative humidity and temperature. Humidity can be defined as the presence of water droplets in air, making it an air-water mixture.

The humid air is moist in nature. In this article we shall study about the effect of temperature on relative humidity. Simply put, relative humidity and temperature are inversely proportional to each other. The reason behind this is discussed in the later sections of this article.

What is relative humidity?

Relative humidity is the ratio of amount of water present in the air water mixture to the maximum of water that can be present in the air water mixture. It represents the relative amount of vapour that can be added up till saturation.

In simple words we can say that it is the measure of how much amount of water is present inside the air water mixture relative to the maximum to the amount of water that can be added to the mixture. Let us see the formula of relative humidity in next section to make things more clear.

Relative humidity formula

We have discussed the definition of relative humidity in the above section. To make the meaning more clear let us have a look at the formula of relative humidity.

The formula of relative humidity is given in the section below-

φ = Pv/Ps

where,

phi is the realative humidity

Pv is the partial pressure of vapour

Ps is the saturation pressure

What is specific humidity?

Specific humidity is different from relative humidity. It is defined as the ratio of amount of water present in the air water mixture to the total mass of the air water mixture.

The formula to calculate specific humidity is given in the section below-

ω = mv/ma

Where,

omega is the specific humidity

mv is the mass of vapour

ma is the mass of air water vapour mixture

Relative humidity and temperature relationship

We all know that water evaporates in the presence of heat or higher temperatures. This phenomenon can be applied to define the relationship between temperature and relative humidity.

When we say that both are inversely proportional to each other, it means one value increases and other decreases. In this case when the temperature increases the value of relative humidity will decrease. And vice versa. The reason being that warm air has more capacity to hold moisture hence the denominator increases. If the air becomes dry then the value of relative humidity also decreases. When the temperature drops, the air becomes wet and hence the value of relative humidity also increases.

Relative humidity and temperature graph

The graph that represents relative humidity and temperature is called as Mollier chart or Mollier diagram. There are many other quantities represented as well.

In the chart we can see that relative humidity curve has a negative slope when we go from right to left. Right to left means the value of temperature decreases. When we go from left to right that is when we increase the temperature, the value of relative humidity also increases.

Relative humidity and temperature formula

To derive a formula between temperature and relative humidity, we use many concepts from thermodynamics mainly- ideal gas law, thermodynamics law, Psychrometrics and conservation of mass.

Using above concepts we get the following formula (The formula is used to calculate the dew point temperature of the mixture)-

Tdp = 4030 (Tdb+235)/4030 – (Tdb+235)lnRH – 235

where,

Tdp is the Dew point temperature

Tdb is the dry bulb temperature

RH is the relative humidity

Relative humidity and temperature chart

We have already discussed about Mollier chart in above section. The Mollier chart represents relationship between various psychrometric properties and dry bulb temperature.

The dry bulb temperature is represented on the x axis or horizontal axis. The value of dry bulb temperature will increase when we move from left to right. The value of relative humidity decreases as we move from left to right or when the dry bulb temperature increases. The reason being that water droplets are evaporated as the temperature is increased. The increased temperature results in dryer air.

relative humidity and temperature
Image: Mollier Diagram

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

Factors affecting relative humidity?

Relative humidity is mainly affected by the temperature and geopgraphic locations. We have already discussed about the effect of temperature on relative humidity. 

The air is wet when temperature is lesser and when the temperature is more the air will become dry as the capacity of air to hold moisture increases. Geographic locations also affect relative humidity. For example, near coastal areas the humidity levels are very high due to evaporation. And places far away from water bodies have lower humidity levels.

What is psychrometry?

Psychrometry is the study of proerties of various properties of air and water vapour mixture. These properties may include specific humidity, dew point temperature, wet bulb temeprature, relative humidity etc.

This branch of thermodynamics holds important applications in refrigeration and air conditioning industry. It is very important to maintain the humidity levels at certain places. Excessive humidity levels can lead to severe damages and failures. Similarly lack of humidity can also harm the system in consideration.

What is wet bulb temperature?

 The name itself suggests that wet bulb temperature is related to wetness of the bulb. It is the temperature that thermometer shows when its bulb is coveres with a wet cloth.  

Due to the wet cloth, the temperature slightly decreases due to heat absorption by water molecules on the cloth. These water molecules are responsible for an increase in relative humidity levels in the system. We should note that wet bulb temperature is always lesser than or equal to dry bulb temperature. The dry bulb and wet bulb temperature are equal at saturation.

What is Mollier diagram?

Mollier diagram is a graphical representation of various psychrometric properties changing with dry bulb temperature.

The dry bulb temperature is represented on the horizontal axis. Other properties are broadly plotted in the graph. Such as specific humidity is plotted as horizontal lines and relative humidity lines are plotted slantly such that the lines bank towards left.

Abhishek

I, Abhishek Khambhata, have pursued Btech 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 performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like minded people.

Recent Posts