On clear, calm nights colder air flows downhill to end up sitting in valley bottoms. If you're watching the weather charts look out for high pressure systems, which appear as a series of widely spaced concentric rings, or isobars, of 1000mbs and more. Depending on the height at which the inversion occurs, hilltops might poke free into the clear air above.

When this colder air is trapped low down by a cap of warmer air the cloud may linger for days. With the long nights and comparatively weak sunshine of late autumn and winter the ground cools, and so too the air at low levels. If the wind brings sufficient moisture off the sea into the weather system it can condense as cloud in the colder air layer. High pressure weather systems - known as anticyclones - bring long periods of stable weather, typically with little wind to break up any cloud.

If you're hoping to catch that special combination of low level cloud and high level sunshine, it pays to have a basic understanding of how it all works: With luck the colder layer will be full of fog or cloud - the magic cloud inversion. But sometimes this standard temperature gradient can be reversed, with colder air low down in the valleys trapped beneath a capping layer of warmer air that sits on the hill tops. In normal daytime conditions the sun warms the ground, and the ground warms the air. As you gain height, walking uphill for instance, the air temperature will tend to drop. 'Generally it is in the winter half of the year when inversions descend onto the British mountains so that higher tops are warmer than lower slopes above a haze or low cloud/fog layer' Geoff Monk, MWIS

Mam tor weather forecast