Air is actually slightly sticky!
( The technical term is Viscous.)

This means that when air flows over a surface like a car the air next to that surface sticks and becomes stationary
.
This affects the next layer away from the surface and so a layer of air develops, the bottom of which is stationary and the top of which is traveling at the speed of the main air flow, with the air in between at some intermediate speed. This region is called the boundary layer and its behaviour greatly affects the performance of a car wing.

If air around a car starts to tumble like white water in a river it is called a turbulent boundary layer and results in reduced downforce and increased drag.

Turbulence can be caused by a number of factors, but one of the most common is distance - all boundary layers tend to become turbulent eventually and separate from the surface over which it flows.

© Cranfield University