What is the exact meaning of CAVOK and CAVU? How are they used by pilots and ATC? Do CAVOK (ceiling and visibility okay) and CAVU (ceiling and visibility unlimited) have formal definitions worldwide?


CAVOK, source: formercaptain.ca

Are they used in ATC communications?
CAVOK is formally defined in ICAO Annex 3 ( APP3-2 2.2 Use of CAVOK ) . You can hear it at the airports where there is no ATIS , when ATC communicates the weather info.
CAVU is not formally defined in Annex 3 .

When the following conditions occur simultaneously at the time of observation:
 a) visibility, 10 km or more, and the lowest visibility is not reported;
 b) no cloud of operational significance;
 c) no weather of significance to aviation as given in 4.4.2.3 and 4.4.2.6;

information on visibility, runway visual range, present weather and cloud amount, cloud type and height of cloud base shall be replaced in all meteorological reports by the term “CAVOK”.