Weather Forecasting

The less one knows about the way the atmosphere works the simpler weather forecasting appears to be. For example if clouds appear in the sky and a light rain begins to fall one might predict that rain will continue throughout the day. This type of weather forecast is known as a persistent forecast. A persistent forecast assumes the weather over a particular geographic area simply will continue into the future.

The validity of persistent forecasting lasts for a few hours, but not much longer because weather conditions result from a complex interaction of many factors that still are not well understood and that may change very rapidly.

A somewhat more reliable approach to weather forecasting is known as the steady-state or trend method. This method is based on the knowledge that weather conditions are strongly influenced by the movement of air masses which often can be charted quite accurately. A weather map might show that a cold front is moving across the great plains of the United States from west to east with an average speed of 10 mph (16 kph). It might be reasonable to predict that the front would reach a place 100 mi (1,609 km) to the east in a matter of 10 hours. Since characteristic types of weather often are associated with cold fronts it then might be reasonable to predict the weather at locations east of the front with some degree of confidence.

A similar approach to forecasting is called the analogue method because it uses analogies between existing weather maps and similar maps from the past. For example suppose a weather map for December 10, 1996, is found to be almost identical with a weather map for January 8, 1993. Since the weather for the earlier date is already known it might be reasonable to predict similar weather patterns for the later date.

Yet another form of weather forecasting makes use of statistical probability. In some locations on Earth's surface one can safely predict the weather because a consistent pattern has already been established. In parts of Peru it rains no more than a few inches per century. A weather forecaster in this region might feel confident that he or she could predict clear skies for tomorrow with a 99.9% chance of being correct.