Fog, mist and haze are not precipitation as they don't fall to the ground. Reduced visibility near the ground is caused by tiny particles suspended in the air. Water droplets that cause a moderate reduction in visibility are called mist. Those causing a serious visibility problem are called fog. Mists and fogs often form over seas, rivers, and lakes. Particles of dust, smoke or salt that affect the clarity of the air are collectively known as haze.

• Fog - visibility below 1,000 m (1,100 yards) - mainly affects aircraft.
• Thick fog - visibility 50-200 m (55 - 220 yards) - dangerous for road traffic.
• Dense fog - visibility below 50 m (55 yards) - seriously disrupts all forms of transport.

There are three different types of fog:

Radiation fog occurs when the land radiates heat into space and the air rapidly cools to the dew point. Radiation fog tends to collect in valleys and other ‘frost hollows' causing motorway pile-ups sometimes because drivers do not slow down enough in foggy conditions.

Sea fog - often known as 'steam' fog (over fresh water) or sea smoke (over the sea) forms when cold air is over much warmer water. This is the same effect as cold air turning to steam over a hot bath or hot sink. There needs to be about 9 degrees C difference for this to happen. True steam occurs when the temperature is 100 degrees C.

Advection fog occurs when warm moist air is cooled by a cooler surface, such as over a cool sea in the spring. Fog banks are common in the northwest Atlantic where they form over cold, shallow, offshore waters, such as the Grand Banks of Newfoundland. It also happens when warm moist air blows over a cold sea or lake.