Barometric pressure (also known as atmospheric pressure) is the force exerted by the atmosphere at a given point. It is known as the weight of the air. A barometer measures barometric pressure. Measurement of barometric pressure can be expressed in millibars(mb) or in inches or millimeters of mercury (Hg). Normal pressure at sea level is 1013.3 millibars or 29.92 inches of mercury. Fluctuations in barometric pressure are usually a sign of weather conditions.