Apart from depressions and anticyclones, the most striking features of any weather map are the bold curved lines carefully adorned with semicircular or triangular barbs. These lines represent fronts. They may be thought of as belts of persistent rain that move steadily across the surface of the globe, usually from west to east or, at a more theoretical level, as the boundary between two masses of air with contrasting qualities.
Fronts are commonly thought of as belonging to one of two varieties. A warm front is a line along which cool air in advance of the front is displaced by the warmer air behind: it is indicated on the weather map by semicircular barbs. A cold front, on the other hand, is one where warm air is replaced by colder air. The cold front on the weather chart generally follows the warm front as they both move eastwards, and the triangular wedge of warm air in between the two is called the warm sector. But if you look at the key to the weather map on this page, you will see that there is a third kind of front - an occluded front. And what, you might well ask, is that?
In the early stages of the development of a typical North Atlantic depression, the whole ensemble - warm front, warm sector, cold front, and the depression itself - usually moves rapidly from west to east. But if you observe the situation over 24 hours or so you will find that the cold front moves rather more quickly than the warm front; it catches up on the warm front as the depression matures, and the triangular warm sector, becoming smaller and smaller, appears to be pushed away to the south. This is the process of occlusion - a word from the Latin which, literally, means a "shutting against".
As occlusion proceeds, the mild humid air in the warm sector is squeezed upwards and out of the way, displaced by the cold air advancing relentlessly from behind. Where it has taken place, the warm sector is no longer apparent at ground level, but there is still a boundary between two air masses - between the two masses of air of differing characteristics brought into contact with each other by the displacement of the warm sector. This boundary is known as an occluded front, and is depicted by alternating triangular and semicircular barbs. Its advance is characterised by thickening cloud, followed by rain, and as it passes a particular spot there is a sudden change to colder, brighter, showery weather.