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The upward search starts from the second lowest model level (Level 136, 31m). Fog, or equivalently cloud in the lowest layer (Level 137, 10m), is not considered when deriving the ceiling.
Fig2.1.5.2-3: 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.
Fig2.1.5.2-4:Schematic representation of difference assessing cloud base and ceiling showing the difference between derivation of Ceiling and Cloud Base.
Fig2.1.5.2-5: Example of differing cloud base and ceiling heights where fog is forecast. Forecasts are for fog (visibility 345m), cloud base (height 29.6m), ceiling (height 8726m) and the corresponding vertical profile. Forecasts VT 00UTC Fri 28 Feb 2025, DT 00UTC Sun 23 Feb 2025. The cloud base identified when the cloud cover is greater than 1% and the moisture greater than 10-6 Kg Kg-1. Cloud cover overnight forecast less than 1% at 10m (Level 137) and separate from forecast of fog. The ceiling identified when the cloud cover is greater than 50% and the moisture greater than 10-6 Kg Kg-1. This corresponds to the moist layer shown on the ENS control (ex HRES) vertical profile (Temperature in Red, Dew point in black). Shading represents the range of temperatures and dew points among the ensemble members.
Convective cloud top height
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