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Fig2.1.5.-3: Schematic representation of difference assessing cloud base and ceiling.  Cloud base is derived by upwardly searching from the surface; the presence of fog implies cloud base at the surface, or possibly a base near 30m in the case of very low Stratus.  Ceiling is derived by upwardly searching from Level136 (about 31m) to avoid allocation of ceiling to the surface in case of fog. This can result in ceiling being quite high (or even there being no ceiling at all) even above surface fog.  Cloud base only requires >1% cloud cover while ceiling requires 50% cloud cover.

Fig2.1.5.-4:Schematic representation of difference assessing cloud base and ceiling showing the difference between derivation of Ceiling and Cloud Base.  

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