The unit used by meteorologists to measure atmospheric pressure is the millibar . Standard sea level pressure is expressed as 1013.2 millibars. Isobars are lines on a weather map that connect places of equal air pressure. A mercury barometer measures air pressure using a column of mercury in a glass tube that is sealed at one end and inverted in a dish of mercury. Standard atmospheric pressure at sea level equals 29.92 inches of mercury.
Wind is the horizontal flow of air from areas of higher pressure to areas of lower pressure.
Geostrophic winds, blow parallel to the isobars and reflect a balance between the pressure-gradient force and the Coriolis effect. Upper-air winds are faster than surface winds because friction is greatly reduced aloft. Friction slows surface winds, which in turn reduces the Coriolis effect. The result is air movement at an angle across the isobars toward the area of lower pressure. In the Southern Hemisphere, the Coriolis effect causes winds to be clockwise around a low and counterclockwise around a high.