GRADIENT WIND

hypothetical wind that accounts for air flow along a curved trajectory. It is an extension of the concept of geostrophic wind-i.e., the wind assumed to move along straight and parallel isobars (lines of equal pressure). The gradient wind represents the actual wind better than does the geostrophic wind, especially when the wind speed and trajectory curvature are large, as they are in hurricanes and jet streams. Whereas the geostrophic wind can be computed from the pressure distribution along a constant level surface or the slope distribution of a constant pressure surface, computation of the gradient wind also involves a knowledge of curvature (changes in wind direction). This information may be derived from the curvature of the isobars. Around a low-pressure centre, the pressure-gradient force directed inward balances the Coriolis force and the centrifugal force, both directed outward; because the Coriolis force acts to the wind's right in the Northern Hemisphere and to its left in the Southern, the wind blows counterclockwise along the curved isobars in the Northern Hemisphere and clockwise in the Southern Hemisphere. In contrast, around a high-pressure centre, the Coriolis force directed inward balances the centrifugal force and the pressure-gradient force, both directed outward; in this case, the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.