Fog, mist and cloud are all formed when air cools to its dew point (the term is self explanatory). Water in the air may condense onto a cold surface, the ground, a house roof or on to small particles in the air (condensation nuclei). At ground level the "cloud" is called fog or mist depending upon the visibility. At sea or for aircraft landing and taking off purposes, a fog is defined as when the visibility is 1000 metres or less. Mist is a visibility between 1000 and 2000 metres. Normally, over land, forecasters use the word "fog" when the visibility is 200 metres or less. This is because a car driver may be fairly happy if he can see over 200 metres while the same is not true for an aircraft pilot landing at Heathrow or the skipper of a boat in mid Channel.