What is required for any type of fog to form?

High relative humidity is essential for any type of fog to form. Fog is essentially a collection of water droplets or ice crystals suspended in the air, and this can only happen when the air becomes saturated with moisture. When relative humidity reaches 100%, the air cannot hold any more moisture, leading to condensation and the formation of fog. This is a fundamental principle in meteorology, as moisture is a critical component in the formation of various types of fog, be it radiation fog, advection fog, or others.

While wind can influence fog characteristics, such as dispersing it or aiding in its formation under specific conditions, the mere presence of wind does not guarantee fog formation. Similarly, while calm winds can facilitate fog development by allowing cooler air temperatures to drop, they are not a strict requirement. Additionally, a cold, clear night is often associated with radiation fog formation, but it is not universally applicable to all fog formations. The presence of high relative humidity remains the constant factor necessary for fog to exist.