In wet weather it can take up to double the distance to stop than it would in dry conditions. In snow and ice it can take up to ten times the distance to stop!
Fog can greatly reduce your visibility. If the visibility is less than 100 metres it states in the highway code that you should put your fog lights on to be more visible.
Also if your car is left outside you need to allow more time to clear the windscreen if there has been a frost or snow.
In all the situations I have stated above you need to reduce your speed according to the road and weather conditions.