Atmospheric Pressure:
As liquids exert pressure on object immersed in it, the air in the earth's atmosphere also exerts pressure on all of us at all times. The pressure exerted by the atmospheric air at any point is equal to the weight of air contained in a column of unit cross-sectional area and extending up to the top of the atmosphere.
The atmospheric pressure at the surface of the earth is approximately 105 Pa. At an altitude of 5 km, the pressure is approximately one half of the pressure at the surface. And, on an altitude of 31 km, pressure is only one percent of the pressure at the surface. The question is: Why does atmospheric pressure decreases with altitude? This is due to the fact that the height of the air column decreases with altitude which results in the decrease of weight of the air in a column of unit cross-sectional area. An instrument called barometer is used to measure the atmospheric pressure. Barometer measures the atmospheric pressure at a point in terms of the height of a mercury column. A standard atmospheric pressure at the sea level is equivalent to the pressure due to a mercury column of height 76 cm or 760 mm. The numerical value of the atmospheric pressure at the sea level is 1.013 × 105 N m - 2 or 105 Pa.
The hydrostatic pressure has some other characteristics. These were discovered on the basis of experimentations and a few laws were formulated. You will now learn some of them such as the Pascal's law and the Archimedes principle.