THE isobars that define the deep depressions on our weather charts at present are, as always, intersected by bold curved lines carefully adorned, with little flags like bunting. These fronts are the sharp dividing lines between two masses of air of greatly differing characteristics and can be thought of as narrow, elongated zones of heavy and persistent rain.
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 semi-circular 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 generally 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 already 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 on the weather map by a combination of the symbols which apply to the other two fronts - alternating triangular and semi-circular barbs. In its effects it is also like a combination of the other two: 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.