Fog is caused by by moisture in the air turning into fine vapour droplets - this happens when the temperature drops to what is called the 'dewpoint'.

'Dewpoint' is simply a term which means that the air cannot hold all of its water as vapour and so the water becomes liquid.

Often you will see mist or fog forming on fields or around rivers, this is because of low ground temperature cooling the air. Effectively, fog is just a cloud at ground level.

The seasons also affect the degree to which fog forms. Autumn is often described as the 'season of mellow fruitfulness' and is characterised by early morning mist.

A more scientific, but less poetic description might be 'the season of radiation fog'! (Caused by water evaporating from land and water and being affected by infra red cooling.)

Fog can be made worse by pollution and so is thicker and more persistent some areas.