Definitions for "Clean air"
When a car is running by itself on the track, it's in "clean air" because the air is not being disturbed by other cars.
This is still air that has not been disturbed by a recently passing car.
Air that isn't turbulent, and thus offers optimum aerodynamic conditions, as experienced by a car at the head of the field.
An operating environment in which no dirt build-up occurs on lenses or reflectors.