Frictional Force

Friction is the force that resists the motion of air as it moves across the Earth's surface. Its most direct effect is on wind speed — it slows the wind down. The strength of this force is not uniform everywhere; it depends heavily on the nature of the surface the air is flowing over.

Friction is strongest right at the ground, where the air is in direct contact with the land. As you move upward, its influence weakens steadily. The book gives a clear vertical limit: friction's effect generally extends up to an elevation of 1 to 3 kilometres above the surface. Above this layer, the wind is essentially free of frictional drag and moves under the influence of the pressure gradient force and the Coriolis force alone. …