Weather is determined by many factors – atmospheric pressure, humidity, temperature and winds. Small, unpredictable variations in any one of these factors can have cumulative and subsequently major effects on the others and the resulting weather systems. Meteorologists are, however, getting better at prediction the weather several days in advance. To do so several requires thousands of measurements of existing weather conditions all over the world, from manned and automatic stations on the ground and at sea, from balloons, and from satellites in space. A variety of measuring instruments are used, the most common being the thermometer and the barometer.
Measurements are fed into a super computer which performs calculations based on mathematical models of typical weather system. It calculates what is likely to happen at a series of points on the ground and in the air. The closer together those points are, the more accurate the forecast will be. A weather model can contain data for millions of point in the atmosphere. Even so, details of the forecast are sometimes wrong, but forecasts can generally predict accurately up to a week in advance.