Which condition is necessary for radiation fog to form?

Radiation fog forms under specific atmospheric conditions typically characterized by high relative humidity. This type of fog occurs during clear nights when the ground loses heat through radiation, leading to a drop in temperature, particularly at the surface. As the air cools, its capacity to hold moisture decreases, resulting in condensation when the air reaches its dew point, thereby creating fog. High relative humidity is crucial in this process because it indicates that the air is already close to saturation with moisture, facilitating the condensation necessary for fog formation.

In contrast, strong winds would disrupt the stable air layer needed for radiation cooling, while high air temperatures would likely prevent the cooling process required for radiation fog to develop. Similarly, foggy skies would typically reduce radiation cooling due to cloud cover, which acts as an insulating layer and prevents the ground from losing heat effectively. Therefore, high relative humidity is vital as it creates the ideal conditions for the air to reach saturation and form radiation fog.