Step 1: Understanding the Concept:
In the upper atmosphere (above the friction layer), air movement is primarily governed by the interaction of two main forces.
Step 2: Detailed Explanation:
The Pressure Gradient Force (PGF) acts from high pressure to low pressure, initiating air movement.
As the air begins to move, the Coriolis Force acts perpendicular to the direction of motion, deflecting the wind.
When these two forces become equal in magnitude but opposite in direction, the wind flows parallel to the isobars. This theoretical wind is called the Geostrophic Wind.
Geostrophic flow occurs when:
\[ \text{PGF} = \text{Coriolis Force} \]
Step 3: Final Answer:
Geostrophic wind is a balance between the Coriolis force and the Pressure Gradient Force.