Counter-air patrol is a type of flying mission for fighter aircraft

Fighter aircraft

A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

 against other fighters, common in World War II and sometimes combined with fighter sweeps (Rhubarb missions) against targets of opportunity

Target of opportunity

Target of opportunity is a military term used to describe a target which, in a combat scenario, is presented or presents itself as a possible target in addition to those targets considered to be primary targets of any particular military operation....

. Counter-air patrol is an attempt to achieve air superiority by maintaining patrols above and around enemy airfields.

Aircraft are vulnerable on take off, fully loaded and working to gain altitude and speed. The aim of the counter-air patrol was to pick off enemy fighters as they took off and during their climb to an operational height. Fighter sweeps were used both to attrit

Attrition warfare

Attrition warfare is a military strategy in which a belligerent side attempts to win a war by wearing down its enemy to the point of collapse through continuous losses in personnel and matériel....

the enemy and to provide cover for bombing raids by forcing enemy fighters to protect their own airfields.