- Due to the heat radiations coming from the sun, a huge amount of water on the surface of the earth gets evaporated.
- When water evaporates, the water vapour moves up and gets mixed up with the atmosphere.
- Hence, the atmosphere consists of water vapours called humidity. The higher the amount of water vapours present in the atmosphere, the higher is its humidity. The moist air is more humid than the dry air.
A) Absolute Humidity
- The absolute humidity is the amount of water vapour present in the cubic metre volume of atmospheric air.
- It is commonly expressed in g/m3.
B) Relative Humidity (R.H)
- The relative humidity is defined as the ratio of the mass of water vapour present in a certain volume of air at room temperature to the mass of water vapour required to saturate the same volume of the air at the same temperature.
- Generally, the relative humidity is expressed in percentage.
- If ‘m’ is the mass of the water vapour actually present in a certain volume of air and ‘M’ is the mass of the water vapour required to saturate the air, then the relative humidity of the air is given by
RH = m/M ×100%
- The relative humidity has no unit. Rather, it is a number which measures the extent to which the air saturates with water vapours.
- An air of 100% humidity means that the air is saturated. In other words, the air cannot contain more water vapour in it.
- Similarly, an air of 50%relative humidity means that the air has only a half of the amount of water vapour which it can contain at the given temperature.
- We know that the higher is the temperature of the air, the more is the amount of water it can contain.
- In other words, an air saturated at a temperature is not saturated at higher temperature.
- This implies that if an air has 100% relative humidity at a given temperature, the air with the same amount of water vapour will not have the same relative humidity at higher tempertaures.
- Hence, it is clear that the relative humidity of an air with a given quantity of water vapour present in it depends upon its temperature.
- Moreover, the pressure of an unsaturated vapour is called partial pressure. The pressure of the saturated water vapour is called saturated vapour pressure.
- The pressure exerted by a vapour is directly proportional to the amount of water vapour present in the air.
- It is, therefore, the relative humidity of air at a temperature can also be defined as the ratio of the partial pressure of water vapour to the saturated vapour pressure at the same temperature.
- That is if ‘p’ is the partial pressure of the water vapour and ‘P’ is the saturated vapour pressure at the same temperature,
RH = p/P × 100%
i.e., RH = Partial pressure of vapour/Saturated vapour pressure at the same temperature × 100%
C) Dew point
- The dew point is the temperature at which the water vapour actually present in the atmosphere is just sufficient to saturate it.
- That is, at dew point, the actual vapour pressure at room temperature is equal to the saturated vapour pressure.
- An unsaturated air can be made saturated either by increasing the vapour content or by lowering the temperature of the air.
- If the temperature of the air is lowered, it cannot contain more water vapours.
- Hence, it saturates at a temperature lower than before.
- The temperature at which a given volume of unsaturated air at a given temperature becomes saturated by without adding more water vapours (or by just lowering the temperature) is the dew point temperature.
- Thus, the relative humidity of the air at a temperature can also be defined as the ratio of the saturated vapour pressure at the dew point to the saturated vapour pressure at the temperature.
i.e., RH = Saturated vapour pressure at dew point/Saturated vapour pressure at room temperature × 100%
- It is important to note that if the temperature of an unsaturated vapour is reduced to the dew point, the condensation of the water vapour present in the air takes place.