Dew and Dew Point

The temperature at which a certain amount of air is saturated with water vapour present in it is called the dew point of that air.

Origin of Dew in the air

Normally water vapour in the air is unsaturated. At night the surface cools by radiating heat. As a result, the atmosphere near the surface also cools.

The amount of water vapour present in the air for cooling does not change. As a result, the cooling air then moves from the unsaturated state to the saturated state by the aforementioned water vapour.

The temperature of the air gradually decreases, eventually reaching a point where the air becomes saturated with water vapour present in it.

As soon as the temperature of the air decreases a little, some amount of water vapour condenses and takes the form of water droplets. This condensed water vapour harbours the dust particles present in the air and forms water droplets. These water droplets are called dew.

Conditions of dew deposition

(i) Cloudless Clear Sky: When there are no clouds in the sky, the surface cools by radiating heat quickly. As a result, the air also cools down quickly. So if the sky is clear, more dew accumulates.

(ii) Still air: If the air is still, the air in contact with a cold object gets a chance to cool down for a longer time. As a result, the probability of the air temperature dropping below the dew point increases.

(iii) Amount of water vapour in the air: If there is a large amount of water vapour in the air, the air becomes saturated if the temperature decreases slightly. As a result, dew accumulation is facilitated.

(iv) Presence of good emitters and poor conductors of heat: Dew forms quickly on objects that are good emitters and poor conductors of heat. Because the material is a good heat-radiator and cools quickly, and being a poor conductor of heat, the material cannot easily absorb heat from the surface. The air cools quickly in contact with such objects. As a result, dew accumulates quickly.

(v) Presence of objects near the surface: Dew accumulates more on objects near the surface. Because cold heavy air can stay longer in contact with such objects. As a result, the temperature of the atmosphere near the object easily drops below the dew point. And, when the object is above the surface, the air in contact with it descends colder and heavier, and the void is filled by lighter air. As a result, the air volume near the object never drops below the dew point. That is why dew accumulates more on grass than on leaves.

Dew Point and Formation of Clouds

When air containing water vapour from the earth’s surface becomes lighter and rises, the air expands in volume due to lower air pressure there. As a result, the volume of that air decreases. Moist air becomes saturated as the temperature drops below the dew point. Then the water vapour in the air accumulates on the dust particles floating in the air and floats in the form of water droplets. Many such water droplets gather together to form clouds.

Causes of fog on ponds in the early morning in winter

Some days in the early morning in winter, the air above the pond water drops below the dew point, as a result, the air becomes saturated with water vapour in the air and the dust particles in the air turn into tiny water particles. Those water particles are fog.