Clear air turbulence (CAT) ahead of an aircraft can be detected in real time by an infrared radiometer. The alert time and reliability depend on the passband of the infrared filter used and the altitude of the aircraft. Preliminary results and analyses show that a nominal passband of 26 to 35 microm appears optimal to alert CAT from 1.5 to 6.0 min ahead of the encounter. The alert time increases with higher altitude as the atmospheric absorption, determining the horizontal weighting, is reduced.