Weather forecasts provide critical information about the weather to come. There are many different techniques involved in weather forecasting, from relatively simple observation of the sky to highly complex mathematical models run on computers. Weather prediction can be for the next day, next week, or next few months. The accuracy of weather forecasts however, falls significantly beyond about 10 days. Weather forecasting remains a complex business, because the weather can be so chaotic and unpredictable.
If weather patterns are relatively stable, the persistence method of forecasting provides a relatively useful technique to predict the weather for the next day. If it is hot and sunny on one day, it is likely to be hot and sunny the next. Unfortunately, in many areas of the world the weather is more unpredictable and changeable than that, particularly in the mid-latitudes where depressions influence much of the weather.
With an understanding of how the air moves and how clouds and rain form, some prediction can be made by simply observing the sky overhead, observing wind direction and noting the temperature and humidity of the air. An abundance of cirrus clouds which gradually thicken to lower level clouds on the horizon for example, is a typical indicator of an approaching depression.
Historically, such observations have led to the development of weather folklore, although many have little sound basis in fact. A commonly known, and frequently accurate saying is, 'Red sky at night, shepherd's delight; red sky at morning, shepherds warning.' When the morning Sun illuminates clouds to the west, this may be an indication of an approaching front and depression. In the evening, the setting Sun in the west will illuminate clouds to the east that may have already passed by.
Better predictions of weather require an understanding of the isobaric patterns associated with fronts and depressions, anticyclones and high-pressure ridges. Meteorologists plot isobaric patterns on synoptic charts. Meteorologists used to plot all their synoptic charts by hand. Since the advent of fast powerful computers however, this task has become much easier. Nevertheless, although organisations like the UK Meteorological Office use supercomputers to predict the weather, television weather presenters still use the isobaric patterns of pressure to describe the movement of depressions and other weather phenomena.