I was asked a question the other day - a simple question, almost trivial to the meteorologist, but one whose answer is by no means obvious to the habitual viewer of the weather presentation on the television: How does the forecaster know where all those fronts should be?
For most of us fronts are just areas of persistent rain which move steadily across the surface of the globe. But to the purist, the rain is almost incidental; in the strict meteorological sense, a front is a dividing line, a boundary between two masses of air of significantly differing characteristics.
Its location can be found by seeking out the lines along which these discontinuities occur.
A working weather chart for a specific time is covered with plotted numbers and symbols, carefully arranged in clusters to summarise the weather at particular spots on the map. The information comes from land weather stations, from ships at sea, oil rigs, weather-buoys and several other sources. It provides information about temperature, pressure, humidity, wind speed and direction, and details of the height, amount and types of cloud, for several hundred places over the area of interest.
The frontal zones are surprisingly well defined. The air on one side of a front is usually warm and humid, that on the other side cool and relatively dry; there is normally a sharp discontinuity of wind - perhaps a strong south-westerly on the eastern side of the front, and a north-westerly behind it; when the isobars are tentatively sketched in, the location of front will be identified by a sharp "kink" in the normally smooth curve of these lines of equal pressure; and, of course, the front itself usually coincides with a line of relatively heavy rain.
Clouds also give clues: ahead of a cold front, for example, a blanket of low cloud covers the sky and visibility is poor; behind it cumulus or cumulonimbus clouds alternate with areas of clear sky, and visibility is good.
Using a previous chart for initial guidance, the forecaster can trace this line of discontinuity in a smooth curve for hundreds of miles across the chart to locate a front stretching, perhaps, from Finland down as far as the Azores.
But, of course, I betray my age. Although a few charts are still drawn by hand, the task has nowadays been largely taken over by computers. The machine goes through much the same logical process as the human, and suggests the likely position of a front. All the human has to do - occasionally - is to tweak the output to make the image more aesthetically pleasing.