THE art of drawing a weather map is less important now than it was some years ago. Computers have taken over much of the routine work, and space age machines produce etchings fully worthy of the great Leonardo. But some of the charts are still drawn by hand, and their execution is a skill in which it takes time to be proficient.
The first attempt is a humbling experience for the novice. He or she is presented with a large map on which the land areas are only faintly outlined, and on which, in a cryptic code, are plotted hundreds of weather observations from all over Europe and the North Atlantic.
The easy bit is sketching in the fronts. Using the previous chart for guidance, they can be spotted by noting sharp variations from place to place in temperature or humidity, and a veering of the wind direction as the front passes each particular weather station their presence is usually confirmed by observations of persistent rain. But then come the isobars - those deceptively innocent lines drawn through points of equal atmospheric pressure.
Isobars are interesting things. Unlike fronts, which have a real existence as a boundary between two masses of air and a physical presence in the form of rain, an isobar exists only in the imagination. It is a theoretical concept, introduced as a mental crutch to help assemble thousands of pressure values into a recognisable pattern. Each is a smooth curve, with no sharp corners except where it crosses a front.
An isobar, moreover, never ends; it is either a closed loop, or else starts at one edge of the chart and meanders around until it "disappears" off another edge. The lines never cross, touch or join; and everywhere along an isobar, the higher pressures are on one side, and the low pressures on the other.
All these rules must be kept in mind, together with the fact that the distance apart of the lines must be proportional to the reported winds and roughly follow their direction.
A weather chart produced by a veteran can be a thing of beauty, with smooth graceful curves executed in bold and confident strokes. The novice's first attempt, by contrast, is inevitably a jumble of wiggly lines - and its centre may well contain a nest of homeless isobars, each with no particular place to go! But the final product will at least serve its purpose, and artistic reward comes in the finishing touches.
The forecaster's magnum opus is completed in living colour, with the cold fronts blue, the warm fronts red, and the occluded fronts a rich imperial purple - "profuse strains of unpremeditated art".