The
geostrophic wind () is the theoretical
wind that would result from an exact balance between the
Coriolis effect and the
pressure gradient force. This condition is called
geostrophic balance. The geostrophic wind is directed
parallel to
isobars (lines of constant
pressure at a given height). This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as
friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the
tropics is close to geostrophic flow much of the time and it is a valuable first approximation. Geostrophic flow in air or water is a zero-frequency
inertial wave.