THE darkest hour, according to the proverb, is always just before the dawn. And similarly it often happens that the heaviest rain comes just before a clearance. The reason can be traced to the structure of a depression, and in particular to the characteristics of the weather fronts embedded in it.
If you glance at a typical depression on a weather map you will usually notice, trailing southwards from the centre, two bold curved lines carefully adorned with little flags like bunting. These are the fronts.
Leading the way on the south eastern side of the low is the warm front, identified by solid semi circular markings or delineated in red when the chart appears in colour.
The cold front, shown ink blue with pointed barbs, trails behind to form a triangular zone between the two, its apex pointing towards the centre of the low. This area between the fronts is called the warm sector, for the obvious reason that the air in this region is usually warmer and more humid than the air elsewhere around the depression.
As a low moves eastwards, its fronts are carried with it and swept anti clockwise around its centre by the spiralling winds. Moreover, the cold front to the rear moves faster than its warm precursor, causing the warm sector to shrink in size as the days go by.
Both warm and cold fronts may be thought of as elongated zones of rain moving steadily across the surface of the globe from west to east, but each has its own generic character.
The rain associated with a warm front sets in gradually, its approach being signalled first by "mares' tails", and then by a gradually thickening veil of cirrus cloud. As time goes by the blanket of cloud becomes thicker and lower until at first gentle, and then steadier, rain begins to fall.
But when the warm front has passed a particular spot, there is seldom a dramatic change: there is a gradual transit ion to damp, cloudy, drizzly weather which lasts until the arrival of the second front.
A cold front is a much more lively entity. Its arrival is often marked by a relatively quick transition from the drizzly warm sector conditions to very heavy, perhaps thundery rain. Now comes the heaviest downpour of the rainy interlude, but its end is sudden.
While the rain still falls, a patch of blue may appear somewhere in the western sky, and the clear area quickly spreads: the rain stops, the sun breaks through, and the wet glistening countryside bakes on a new look of brightness and life. The cold front has passed by.
