Most people are interested in what the weather will be tomorrow or in the next few days. Predictions of the weather for up to a week in the future are called short-term forecasts. Meteorologists also try to make long-term forecasts of the weather for a month, a season, or a whole year.
In earlier times, before the telegraph and the telephone were invented, weather observations from faraway places could not be collected in one place soon after they were made. In those times, the only way of predicting the weather was to use your local experience. Given the weather on a particular day, what kind of weather usually follows during the next day or two? As you can imagine, the success of such forecasting was not a lot better than making a random guess.
Beginning in the late 1800s, weather services began to be able to gather weather data from weather stations located all over the country. That allowed meteorologists to plot weather maps and see weather systems moving from place to place. That improved the accuracy of forecasts.
During the 1900s, meteorologists developed even better tools for observing and predicting the weather. Special instruments measure weather in the atmosphere far above the ground, and satellites orbiting the Earth send back images of the weather over large areas. In addition, computer models are now being developed for weather forecasting. In a computer model, the important processes of the weather are built into the model. The model starts with the present weather and tries to simulate how the weather will develop in the future. Today's computer models do a very good job of predicting the weather for the next few days.