Weather forecasters view the world in a strange, surreal sort of way. They see it as an esoteric jumble of numbers, dots and other hieroglyphic symbols, carefully arranged in clusters on the weather chart. Each cluster is about a centimetre square, summarising the weather at a particular spot, and on a large-scale map of Europe there may be half a dozen of such groups of figures plotted over the faint outline of our island.
Off the coast there may be similar groups depicting observations from lighthouses, oil rigs, ships at sea and several weather-buoys.
The notion of depicting the weather on a map in terms of numbers and symbols instead of words is said to have originated with Johann Heinrich Lambert in the late 18th century. Lambert, who is chiefly remembered for the conformal map projection which bears his name, also had an interest in meteorology, and saw the advantages of the system he devised as twofold: it was independent of language, and it was economical with space.
Lambert's selection of symbols was relatively small. He used the equals sign for cloudy, a broken oblique stroke, rather like the forward slash used in computer-speak, for rain, a row of kisses, as it were, for snow, and two dots arranged as a colon to represent fog. A century later, in 1873, as meteorology became more organised and international, the advantages of Lambert's innovation were even more apparent, and a Meteorological Congress held in Vienna that year adopted a set of standard symbols that are still with us with only minor changes.
Readings of pressure, temperature and humidity require little ingenuity, since they can be represented easily by a number, and numbers are intrinsically concise and international. Other elements can be illustrated by an almost intuitive metaphor, like a zig-zag sign for lightning and an arrow for the wind, where the number of feathers gives an indication of its strength.
But the symbols for most of the more common weather phenomena must be learned.
A single dot near a weather station indicates the presence of light, intermittent rain. If there are two dots, the rain is known to be continuous, and three or four dots, in specific patterns, indicate rain of progressively greater intensity. The same convention applies to drizzle, where the symbol is a comma, and snow, when an asterisk is used. Horizontal lines in various configurations are used for haze or fog, an inverted triangle represents a shower, and there are more than 30 symbols, each of them pictorially mnemonic, to represent as many different types of cloud.