The two main types of fog. Most common over land is Radiation Fog which occurs on clear nights with light winds. Radiation from the ground escapes out to space, the ground cools and, in turn, cools the air in contact with it. On an absolutely still night, condensation will occur on the ground to form dew. With slight air movement, sufficient condensation forms on condensation nuclei to form very small droplets of water - mist or fog. If the wind increases or a layer of cloud comes over, then the fog is likely to clear.