Annex 3
4.6.5 Clouds
4.6.5.1 Cloud amount, cloud type and height of cloud base shall be observed and reported as necessary to describe the clouds of operational significance. When the sky is obscured, vertical visibility shall be observed and reported, where measured, in lieu of cloud amount, cloud type and height of cloud base. The height of cloud base and vertical visibility shall be reported in metres (or feet).
4.6.5.2 Recommendation.— Cloud observations for local routine and special reports should be representative of the runway threshold(s) in use.
4.6.5.3 Recommendation.— Cloud observations for METAR and SPECI should be representative of the aerodrome and its vicinity.
Also a METAR is usually only given once hourly unless regional agreements require it half-hourly. The conditions to issue a Speci are usually "when there are 5 oktas or more of total cloud covering the celestial dome* below the highest alternate minimum or 1500 ft, whichever is higher" but not all units monitor for changes depending again on where the Met Office is (or if there even is one on the aerodrome.