Incidentally, mercury, the heaviest substance available that remains a liquid at ordinary temperatures, permits the barometer to be of a manageable size. Toricelli could have used water in his barometer, but at sea level it would have been 34 feet tall. The atmospheric pressure observation is the principal weather element used in forecasting weather. Significant changes in pressure usually precede a change in weather patterns. Large numbers of observations, preferably global, are required to prepare a weather forecast.
Pressure is reported in millibars, which is a more precise scale than inches of mercury.
For example, 1013.2 millibars is equal to 29.92 inches of mercury at sea level. Because pressure varies with elevation, observations made by weather stations are mathematically reduced to sea level to establish a universal reference level. The lines on a weather map that connect weather stations of equal pressure are called isobars and outline significant areas of high and low pressure as shown in the illustration.