The Geostrophic Wind

It has been noted many times this quarter that on upper air charts the wind blows parallel to the height lines, with highs to the right of the wind and lows to the left of the wind in the northern hemisphere. These winds are in geostrophic balance.

Away from the ground, there are two majore horizontal forces that act on an air parcel: the Pressure Gradient Force (PGF), and the Coriolis Force (CF). The PGF is always perpendicular to the isobars or height contours, and directed toward low pressure/height. The CF always acts perpendicular to the wind, to the right of the wind in the northern hemisphere and to the left of the wind in the southern hemisphere.

Geostrophic balance occurs when the PGF and the CF are equal strengths and opposite directions. The forces on the geostrophic wind (Vg) will always be represented as outlined below.














The following Lab Worksheet 1 provides an opportunity to practice some of these concepts.

Near the surface, the PGF and the CF are just as important, but the frictional force must also be considered. When friction is added to the force balance, the wind is not considered to be in geostrophic balance anymore. The frictional force slows the parcel's motion. It is, therefore, always opposite to the motion of the parcel. Since friction causes the parcel to slow down, friction also causes the CF to get weaker (recall, the CF is proportional to the speed of the parcel). The PGF is, therefore, more than the CF, making the winds angle more toward lower pressure/heights than geostrophic winds.

The following Lab Worksheet 2 provides an opportunity to practice some of these concepts.