These are formed when warm air rises over a mass of cold air.
As the air lifts into regions of lower pressure, it expands, cools and condenses the water vapour as wide, flat sheets of cloud.
Warm fronts are shown on synoptic charts by a solid line with semicircles pointing towards the colder air and in the direction of movement.
On coloured weather maps, a warm front is drawn with a solid red line with red semicircles.
These are usually associated with depressions.
A cold front is the transition zone where a cold air mass is replacing the warmer air mass.
The cold air is following the warm air and gradually moves underneath the warmer air.
When the warm air is pushed upwards it will rain heavily.
Often more rain will fall in the few minutes the cold front passes than it will during the whole passage of a warm front.
As the cold front passes, the clouds roll by and the air temperature is cooler.
Cold fronts are shown on synoptic charts by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement.
On coloured weather maps, a cold front is drawn with a solid blue line with blue triangles.
These occur at the point where a cold front takes over a warm front or the other way around.
If a cold front undercuts a warm front it is known as a cold occlusion and if the cold front rises over the warm front it is called a warm occlusion.
Occluded fronts bring changeable weather conditions.
On a synoptic chart, occluded fronts are represented by semicircles and triangles positioned next to each other.
The triangles are in blue and the semicircles are in red, or both are purple (mixing both red and blue colours together).