Fog, dew and clouds are all related. In order for fog to form, the moisture must leave the air and condense. This means it must be cooled in some way, because cooler air cannot hold as much moisture as warm air. When the air is cooled below a certain point, called the dew or saturation point, then fog starts to form. One of the conditions in which fog forms is when a mass of warm air passes over a cold area of water. Or it could be the opposite, with cold air passing over warm water. The currents of warm and cold air mix gently and you get those familiar fogs which seem to hang in mid-air over a body of water.
Fact file
Tiny water droplets condensing from moist air cause both mist and fog. Mist is not as dense as fog. It commonly occurs on calm, clear nights when heat rises. This forms a thin layer of mist close to the ground.
resources: Tell me why (Chancellor Press)
image 1: http://ak4.picdn.net/shutterstock/videos/5703524/thumb/1.jpg
image 2: https://upload.wikimedia.org/wikipedia/commons/2/27/Spider_web_with_dew_drops.jpg
