4.1. Atmospheric Pressure

The atmosphere is held on the earth by the gravitational pull of the earth. A column of air exerts weight in terms of pressure on the surface of the earth. The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. Pressure is normally measured in millibars or pascals and spatial variations of pressure are depicted on maps by means of isobars, which are lines connecting places having the same barometric pressure. The actual pressure at a given place and at a given time fluctuates and it generally ranges between 950 and 1050 millibars. Air pressure is measured with the help of a mercury barometer or the aneroid barometer.

The gradual change of pressure between different areas is known as the barometric slope or pressure gradient. The closer the isobars are together, the greater the pressure gradient; for example, widely spaced isobars indicate a weak pressure gradient.