Fog
	The official definition of fog is when visibility is reduced to less than 1000 metres. It is composed of tiny water droplets suspended in air. Thicker fog occurs in industrial areas due to pollution attaching onto the water particles, which is known as smog. Freezing Fog This occurs when water droplets in the air are at a temperature below freezing.

Radiation Fog

This takes place at night when the ground loses its heat and cools the air to saturation point. Fog forms near to the ground because the air near the surface falls below its dew point.

Water vapour condenses and attaches onto dust and other particles, forming cloud at ground level which thickens overnight. It occurs when there are light winds and clear skies on long nights. Some fog will be burnt off at dawn and when the air eventually warms up it evaporates to produce tiny droplets of moisture.

Sea /Advection Fog

Moist air is cooled by travelling over a cold sea and becomes saturated. Fog may then form in coastal areas. This particularly occurs in spring and summer and is called advection fog because it is blown by the wind. The fog drifts over a previously clear area, moving sideways as opposed to convection fog which has an upward movement.

Rain clears fog. This can result by either the fog clearing before rain arrives or falling rain serving to improve visibility. In addition the accompanying wind with rainfall can clear fog.