Weather forecasts provide critical information about the expected weather conditions for a certain period of time ahead and a certain location. Forecasts may be also prepared for a particular area, or route between two locations. There are many different techniques involved in weather forecasting, from relatively simple observation of the sky to highly complex numerical models run on powerful supercomputers, such as the famous Cray computer.

Weather forecast can be for the next one to five days (short-term forecasts), next week or ten days (medium-range forecasts), or next few weeks or months (long-range forecasts). Last but not least there are seasonal forecasts , trying to predict the expected average weather conditions for a wh ole season. 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.

Short-term forecasts, e.g. WeatherOnline's UK & Ireland summary, are carried out by synoptic meteorologists, based on the standard methods of weather analysis and numerical weather prediction. Medium- and long-range forecasts are usually based on ensemble-technique forecasts. A computer simulation is run several times with slightly different initial conditions. Ensemble forecasts are helping to estimate the probability or confidence of a particular forecast.

With a little understanding of how the air moves and how clouds and rain form, some prediction can be made yourself by simply observing the sky overhead, observing wind direction and noting the temperature and humidity of the air. For example, an abundance of thin high clouds (cirrus clouds) which gradually thicken to lower level clouds on the horizon is a typical indicator for the warm front of an approaching depression, and usually means rain and wind.

Better predictions of weather require an understanding of the isobaric patterns associated with fronts and depressions, anticyclones and high-pressure ridges. Isobars are lines of equal pressure plotted on a synoptic chart. Isobaric patterns of pressure are still the most common way to understand the movement of high- and low-pressure areas, indicating an approaching front or depression.