Correct Option
The Earth's surface radiates longwave infrared radiation at night, leading to cooling. On clear nights, this radiation escapes into space, causing the ground to cool sufficiently for dew to form if atmospheric moisture is present. On cloudy nights, clouds act as a barrier. They reflect a significant portion of the outgoing longwave radiation back towards the Earth's surface. This reflection reduces the net heat loss from the ground, maintaining higher surface temperatures. Consequently, the ground temperature does not drop below the dew point, preventing the condensation of water vapor into dewdrops.
Incorrect Options
Option (a): While clouds do absorb some outgoing longwave radiation, their primary role in preventing dew formation on cloudy nights is the reflection and re-emission of this radiation back towards the Earth's surface. This reflection is the dominant mechanism that reduces net heat loss and keeps surface temperatures above the dew point.
Option (c): This statement is incorrect. On cloudy nights, the reflection and re-emission of outgoing longwave radiation by clouds significantly reduce heat loss from the Earth's surface. This mechanism helps maintain higher surface temperatures compared to clear nights, thereby inhibiting the formation of dew.
Option (d): Wind deflection by clouds is not a direct factor in the process of dew formation. Dew formation primarily depends on the cooling of a surface below the dew point temperature, which is influenced by radiation balance and atmospheric moisture content, not wind deflection.