Iridescence in fog. This optical phenomenon is caused by diffraction of sunlight by water droplets in the fog. Iridescence occurs when the Sun is not directly behind the water droplets. Fog is caused by the cooling of warm, moist air in the lowest layers of the atmosphere. Water vapour condenses into droplets as the air cools. Morning fog clears as the air temperature rises. This is an example of supercooled fog. Photographed in Finland.