AIM 7-1-23 Clear Air Turbulence (CAT) PIREPs
Chapter 7. Safety of Flight · Section 1. Meteorology
A reference copy of the official AIM text for study. It is not legal advice or an FAA interpretation; always confirm against the current official source.
CAT has become a very serious operational factor to flight operations at all levels and especially to jet traffic flying in excess of 15,000 feet. The best available information on this phenomenon must come from pilots via the PIREP reporting procedures. All pilots encountering CAT conditions are urgently requested to report time, location, and intensity (light, moderate, severe, or extreme) of the element to the FAA facility with which they are maintaining radio contact. If time and conditions permit, elements should be reported according to the standards for other PIREPs and position reports.
REFERENCE-
AIM, Para 7-1-21, PIREPs Relating to Turbulence.