Tiempo Sudamérica

How to get maximum benefit from a weather forecast

The fact that both wind and weather are related to the pressure field is the main reason why weather charts are so much as part of weather forecasting. The isobars and fronts provide the skeleton of a weather analysis and the satellite pictures help to put flesh on the bones. But a good analysis is only the beginning of a forecast. The next step is prediction of the movement and development of the pressure systems and this is where the difficulties start, for there are no hard and fast rules and every day brings a different situation.

The depression that caused all the trouble during the Fastnet yacht race in August 1979 illustrates the scale of the problem. Forty eight hours before the disaster, the low responsible was a wave depression moving quickly East, just to the South of Newfoundland, as a small and harmless feature. During the eighteen hours before it reached Southwest Ireland the centre deepened rapidly; this increased the pressure gradient (i.e. brought the isobars closer together), strengthened the wind to gale, locally storm force, and generated the dangerously high seas that had such tragic consequences.

By no means all depressions move as fast or deepen as explosively as did the Fastnet storm, but this example does show clearly that pressure systems on the other side of the Atlantic can have a direct effect on European weather within a couple of days. Indeed, such systems can also have very significant indirect consequences. If a strange current of warm air is swept Northwards across Labrador, this can induce a compensating plunge of cold air to develop Southwards across the British Isles same 48 hours later. For successful weather forecasting several days ahead it is clearly necessary to know what is happening over a very large area indeed. On the much shorter time scale of a few hours, a different approach is possible. Farmers and fisherman have scanned the sky and felt the wind for thousands of years and a great deal of useful weather lore stems from their careful observations. There is a limit to what can be achieved by these methods alone, but equally, no one would pretend that prediction techniques based on fronts and pressure systems always give a perfect solution. For anyone really interested in the weather, an optimum strategy is one that marries the two approaches and thus benefits from the strengths of both.

Professional weather forecasts are now widely available in the Press, via the Coast Guard, on Radio and on Television. These are based on observations gathered from all quarters of the globe and make use of computer calculations that help to elucidate the complex dynamic processes which control the development and movement of pressure systems. Many of the errors which occur, though perhaps having major consequences for the weather at a particular point, stem from comparatively minor errors in the predicted pressure field; the pattern is usually broadly correct, but systems may move a little faster or slower than expected, or in a slightly different direction, leading to small but vital errors in the forecasted timing or position.

This is where the keen weather watcher can best make his own contribution. Accepting the framework affronts and pressure systems set out in forecast weather maps or explained in many radio presentations, he can monitor how the situation evolves by observing the clouds, the wind and above all the barometric characteristic. He can then make his own adjustments to the professional forecasts, and also feed in all his own local knowledge. In this way he will indeed get maximum benefit from weather forecasts.