Definition
The resistive force created where moving air contacts the Earth's surface, slowing the wind near the ground and altering its direction relative to the wind aloft. Friction is significant in roughly the lowest 2,000 feet of the atmosphere, known as the friction layer or boundary layer.
Plain English
The drag the ground puts on the wind. Trees, buildings, hills, and even the surface of the ocean rub against the moving air, slowing it down and bending its direction near the surface.
Context Anchor
Seen in aviation weather discussions about surface wind, wind aloft, and why wind near the ground may not match the wind above the surface.
Derivation
From Latin 'frictio,' meaning rubbing. The word still carries that everyday sense here: air rubbing against the surface it flows over.
Why Pilots Care
It reduces wind speed near the surface and causes winds to cross lines of equal pressure at an angle, affecting low-level flight planning and navigation.
Grounding Statement
Picture wind blowing across a forest versus across a smooth lake. The forest slows the wind much more. That slowing effect is frictional force at work.
Intuition Check
Frictional force does not mean only brake friction or tire friction. In this weather context, it means the slowing effect of the Earth’s surface on moving air.
Example Sentence 1
Frictional force is why the wind reported at the surface is often weaker and from a slightly different direction than the wind a few thousand feet above.
Example Sentence 2
When flying close to the surface, a pilot notices that frictional force makes the wind weaker than the wind higher up.