What is the main factor that contributes to the formation of radiation fog?

The formation of radiation fog primarily results from long wave radiation cooling at night. As the ground loses heat through radiation after sunset, the air close to the surface also cools down. When the temperature of the air near the ground drops to the dew point, the moisture in the air condenses into tiny water droplets, creating fog. This process is intensified under clear skies with little to no wind, as there are fewer disturbances to mix the air, allowing the temperatures to fall more rapidly.

While high humidity levels can certainly contribute to fog formation, they are not the sole or main factor. Instead, the cooling process due to long wave radiation is critical in reaching the dew point necessary for condensation and fog formation. Warm temperatures, particularly when combined with high humidity, do not lead to fog under normal circumstances because they often prevent the air from reaching the necessary cooling point. High winds would typically disperse moisture and keep the air mixed, which would inhibit the cooling process necessary to form radiation fog.