Air pressure, the force exerted on a mass by the weight of air particles, is measured in either inches of mercury or in millibars, represented by inHg and mb, respectively. One standard atmosphere, represented by the symbol atm, is equal to 29.92 inHg, or 1013.25 mb.
High air pressure in excess of 29.92 inHg, or 1013.25 mb, indicates a high concentration of air particles at the surface. In meteorology, high air pressure systems are associated with clear weather conditions. Low air pressure, or air pressure below 29.92 inHg, or 1013.25 mb, indicates the lifting of air particles away from the surface. In meteorology, low air pressure systems are associated with poor weather conditions. For example, in the eye wall of a hurricane, the air pressure can dip to as low as 990 mb.