The third force, which factors into wind's speed and direction is friction. Even though air is a fluid, the air molecules still rub across the surface of the Earth. Air is also channeled and diverted by buildings, trees, and hills. All these effects cause friction, a rubbing of the air molecules across the surface. Air well above the surface experiences little friction, while air nearer the surface experiences more friction. The layer where air is most effected by friction and the surface is called the boundary layer.

Friction has two effects on the wind. Friction opposes the direction of motion by acting opposite to the flow of air. The force of friction changes the air's speed.

Friction acts to slow the wind by dragging across the surface (Fig. 4.27). The trees, buildings etc. slow the wind.

Fig. 4.27 Profile of wind speeds in the lower atmosphere. Friction causes slower speeds near the surface. Upper level winds experience little friction.
Study Question 4.15
Using the Coriolis equation from the previous page, what happens to the Coriolis force when friction slows the wind speed?
Increases Coriolis force
No change
Decreases Coriolis force

Since the Coriolis force is reduced by the wind speed decrease caused by friction, the Coriolis force and pressure gradient force will not balance each other. The balance between the pressure gradient force and the Coriolis force that existed in geostrophic flow is overcome (Fig. 4.28). The imbalance will cause the pressure gradient force to dominate produce the flow seen at the surface around high and low pressure areas (which will be discussed in Lesson 8b. Here winds blow across the isobars in toward low pressure areas and away from high pressure areas.

Fig. 4.28 As friction slows the wind speed, the pressure gradient force (PGF) and Coriolis force (CF) no longer are balanced. When this happens, winds blow across the isobars.

This combination of forces occurs only in the scale of motion relating to the synoptic-scale and larger. This scale refers to the scale of low and high pressure systems and larger. The Coriolis force only works here because of the large-scale motion. When dealing with smaller scale wind flows, pressure gradient is the main driving force. Look back at Figure 4.24 to see if this applies in reality.