Previous Year Question

Why are dewdrops not formed on a cloudy night?

Clouds absorb the radiation released from the Earth’s surface.

Clouds reflect back the Earth’s radiation.

The Earth’s surface would have low temperature on cloudy nights.

Clouds deflect the blowing wind to ground level.

Correct Answer: B

Detailed Solution

For dew to form, the ground and objects in contact with it must cool through radiational heat loss to the dew point.

On a clear night:

Earth’s surface emits long-wave infrared radiation towards space.

With a clear sky, a substantial part of this radiation escapes to space.

The surface can cool rapidly and reach the dew point.

Water vapour in the air touching the surface then condenses as dew on grass, leaves and other cold objects.

Calm winds favour dew formation because strong winds mix the cold surface air with relatively warmer air above it.

On a cloudy night:

Clouds absorb outgoing long-wave radiation and re-emit a part of it back towards Earth.

This reduces heat loss from the surface and prevents it from cooling sufficiently.

Since the surface temperature generally does not reach the dew point, dew formation is prevented or greatly reduced.

The expression “reflect back” in option (b) represents the absorption and downward re-emission of infrared radiation by clouds. This downward radiation reduces nocturnal surface cooling.

Why not option (a)? Clouds do absorb terrestrial radiation, but the immediate reason for maintaining a warmer surface is that part of the absorbed energy is re-emitted downwards. Option (b) captures this return of energy towards the surface.

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