The formation of clouds occurs with the condensation of water vapour into visible water droplets or ice crystals. When a parcel of air holds as much water vapour as it can carry at a particular temperature and cannot hold any more in vapour form, it condenses into liquid (water droplets) or solid (ice crystals) form. This is known as the saturation point, and it can be achieved in two ways:

1. Moisture accumulates in the air due to evaporation to the point that it can hold no more.
2. The air temperature reduces until it reaches the dew point, i.e., the temperature at which condensation occurs, and it cannot hold any more water. Typically, warmer air can ‘hold’ more water vapours. Hence, as temperature decreases, the air’s capacity to ‘hold’ water vapour also reduces, and condensation occurs to liquid (water droplets) or solid (ice crystals) forms.

Generally, the formation of clouds takes place by the second method. It is related to the heating of air in the lower parts of the atmosphere and its expansion as it rises due to lower atmospheric pressure. The energy utilised for expansion helps reduce the temperature of air and causes it to cool. In general terms, the air cools by 1°C for every 100 metres that it rises. 

Hence, as air rises vertically, its capacity to hold water vapour decreases, and condensation occurs. The height at which the formation of clouds takes place on reaching the dew point is called the condensation level. 

Rising and cooling of air

Five factors govern the rising and cooling of air.  

1. Surface heating: The air in contact with the ground heats up as the ground gets heated due to the sun, and the heated air rises and expands. The rising columns of air are known as ‘thermals’. 
2. Topography: When a mass of air is forced over a barrier of mountains or hills from a lower elevation; it is also known as ‘orographic uplift.’
3. Frontal: When a mass of air in contact with the ground heats up and rises over a mass of cool air. The boundary between the two air masses is known as a ‘front’.
4. Convergence: When streams of different air masses flowing from various directions meet and are forced to rise. 
5. Turbulence: On the vertical ascension, the air masses can experience sudden changes in wind speeds, generating turbulent eddies in the air.

The atmosphere consists of millions of tiny salt, dust or smoke particles that act as condensation nuclei for the water vapours to condense when the saturation point is reached. 

Latent heat of condensation

A cloud can be defined as an accumulation of minute water or ice droplets or a mixture of both floating in the air. When a parcel of air holds as much water vapour as it can carry at a particular temperature and cannot hold any more in vapour form, it condenses into liquid (water droplets) or solid (ice crystals) form and forms clouds.

As the moisture in clouds increases gradually, the clouds become heavier and return to the earth’s surface due to gravity. This phenomenon is called precipitation (rain, snow, sleet, hail, etc.). Precipitation occurs mainly through two phenomena. 

1. When ‘cold clouds’ are composed of a mixture of ice crystals and water droplets, they experience an imbalance in the vapour pressure of water, and the water droplets condense to form ice crystals. Subsequently, the cloud becomes heavier, succumbs to gravity, and falls on the earth’s surface.
2. When the collision of water droplets in warm clouds occurs and causes a change in their electrical charge, water droplets that carry opposite charges attract and agglomerate till they become heavy enough to fall on the earth’s surface.

Water present in the atmosphere exists in all of its three phases: gaseous (as water vapour), liquid (as water droplets) and solid (as ice crystals). Among these three states, water in its gaseous form acquires the highest internal energy, followed by the liquid state and then the solid state. When any substance in the higher internal energy phase transforms to a phase having lower internal energy, it is accompanied by the release of energy; when any substance in the lower internal energy phase transforms to a phase with higher internal energy, it is accompanied by energy absorption. 

The absorption or release of energy by the water vapours present in the atmosphere as it changes into its phases is known as latent heat of condensation. 

When a parcel of air holds as much water vapour as it can carry at a particular temperature and cannot hold any more in vapour form, it condenses into liquid (water droplets) or solid (ice crystals) form). The energy released during the condensation of water vapour into water droplets and ice crystals and vice versa is known as latent heat of condensation. 

Latent heat of condensation is crucial as it is not only associated with the phase change of water in the atmosphere but also affects the surrounding environment. For example, when water vapours undergo condensation or freezing, energy is released into their surrounding environment. The surrounding environment absorbs this energy, and it warms up. Similarly, when water molecules undergo evaporation or sublimation, they absorb energy from the surrounding environment. The absorbed energy changes the phase of the water molecules, and since the surrounding environment loses energy, it cools down. 

Latent heat of condensation in thunderstorms and hurricanes 

Latent heat of condensation plays a significant role in causing thunderstorms and hurricanes. When warm air rises from the earth’s surface, the water vapours it contains condenses onto tiny salt, dust or smoke particles that act as condensation nuclei when the saturation point is reached. Therefore, the water vapours condense, and the latent heat of condensation is released into its surrounding environment. The energy released de-stabilises the surrounding air around another cloud droplet, and it warms up. This warm air around the cloud droplet then rises and condenses. The height of the cloud, therefore, increases gradually. If the atmosphere becomes highly unstable, thunderstorms or hurricanes could be formed.

Conclusion

The absorption or release of energy by the water vapours present in the atmosphere as it changes into its phases is known as latent heat of condensation. Water vapour present in the atmosphere acts as a greenhouse gas and plays a significant role in the formation of clouds.