ICAO Annex 3 Chapter 4 para 4.13.2. states:
When the following
conditions obtain simultaneously at the time of observation:
a. visibility, 10km or more;
b. no cloud below 1,500m (5,000ft) or below the highest minimum
sector altitude, whichever is greater, and no cumulonimbus;
c. no weather phenomena of significance to aviation;
information on visibility, runway
visual range, present weather and cloud amount, type and height shall be
replaced in all meteorological reports by the term “CAVOK”.