Cobalt wrote:
Really? That sounds wrong, it means that I would have to expect a possible1,500ft overcast even if CAVOK is forecast. That makes no sense and is not done that way anywhere.
It may make no sense, but it is true. The EASA rules for change groups (which must be based on ICAO) are:
AMC1 to MET.TR.220(f) Aerodrome forecasts
TAF — USE OF CHANGE GROUPS
The criteria used for the inclusion of change groups in TAF or amendments to TAF should be based on the following:
[…]
(g) when the height of base of the lowest layer or mass of cloud of BKN or OVC extent is forecasted to lift and change to or pass through one or more of the following values, or when the height of the lowest layer or mass of cloud of BKN or OVC extent is forecasted to lower and pass through one or more of the following values:
..(1) 100, 200, 500 or 1 000 ft (30, 60, 150 or 300 m); or
..(2) 1 500 ft (450 m) in cases where significant numbers of flights are operated in accordance with the visual flight rules;
(h) when the amount of a layer or mass of cloud below 1 500 ft (450 m) is forecasted to change:
..(1) from NSC, FEW or SCT to BKN or OVC; or
..(2) from BKN or OVC to NSC, FEW or SCT
It means (assuming VFR or IFR below minimums) you can avoid a SCT cloud (not reported in the TAF since it’s not a ceiling) but cannot avoid the rain that’s under it. I agree it makes little sense, although it’s easier to get caught in a rain shower than inside a cloud.
Airborne_Again wrote:
It may make no sense, but it is true. The EASA rules for change groups (which must be based on ICAO) are:
Thanks for that! Something that I never knew. It explains why 1400 and 1600 cloud bases are so common in TAFs!
Astonishing, and I bet very few people knew that…
