what constitutes a ceiling
Not to be confused with Cloud top. In aviation, ceiling is a measurement of the cloud base height relative to the ground. Ceiling is reported as part of the METAR (METeorological Aviation Report) used for flight planning by pilots worldwide. A ceiling listed as unlimited means either that the sky is mostly free of cloud cover, or that the cloud is high enough not to impede Visual Flight Rules (VFR) operation. In a METAR, the lowest BKN or OVC layer specifies the cloud ceiling.