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Atmospheric Pressure. The pressure exerted by the earth's atmosphere at any given point. The standard or normal atmospheric pressure (at sea level) is equivalent to the pressure produced by a column of mercury 29.9 in. (760 mm) high. This pressure decreases at higher elevations and with changing weather patterns (high or low pressure systems). Water boils at 212¦F (100¦C) at normal atmospheric pressure and at somewhat lower temperatures as the pressure is reduced.