When do you see clouds?

Fog is caused by a drop of temperature when the air is moist. Air always has some moisture in it. On a warm day there will be quite a lot of moisture in the air caused by evaporation of surface water or transpiration from plants. However, if the temperature drops the moisture starts to condense into droplets big enough to cause light scatter. We call this size water droplets low to the ground - fog. Early morning fog often clears as the sun hits it due to the re-evaporation of the water.