text: The basic principles of physics and the Coriolis effect define an approximate geostrophic wind or gradient wind, balanced flows that are parallel to the isobars. Measurements of wind speed and direction at heights well above ground level confirm that wind matches these approximations quite well.  However, nearer the Earths surface, the wind speed is less than predicted by the barometric pressure gradient, and the wind direction is partly across the isobars rather than parallel to them.  This flow of air across the isobars is a secondary flow'., a difference from the primary flow which is parallel to the isobars. Interference by surface roughness elements such as terrain, waves, trees and buildings cause drag on the wind and prevent the air from accelerating to the speed necessary to achieve balanced flow.  As a result, the wind direction near ground level is partly parallel to the isobars in the region, and partly across the isobars in the direction from higher pressure to lower pressure.
  As a result of the slower wind speed at the earths surface, in a region of low pressure the barometric pressure is usually significantly higher at the surface than would be expected, given the barometric pressure at mid altitudes, due to Bernoullis principle.  Hence, the secondary flow toward the center of a region of low pressure is also drawn upward by the significantly lower pressure at mid altitudes.  This slow, widespread ascent of the air in a region of low pressure can cause widespread cloud and rain if the air is of sufficiently high relative humidity.
  In a region of high pressure (an anticyclone) the secondary flow includes a slow, widespread descent of air from mid altitudes toward ground level, and then outward across the isobars.  This descent causes a reduction in relative humidity and explains why regions of high pressure usually experience cloud-free skies for many days.