Astonishing, and I bet very few people knew that…
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!
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.
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
Peter wrote:
How is the SHRA in the bold section possible when no cloud is forecast for that period? Well, CAVOK is 5000ft AAL (or MSA if above 5000ft) but EGKK is almost at MSL.
The TAF at first describes the expected condition at the start of the forecast period.
0506/0612 VRB03KT CAVOK
Any change to that has to be above a certain threshold level. SHRA is a significant weather condition and requires a change group.
PROB40 TEMPO 0512/0518 7000 SHRA
Therefore implies that there is a 40% chance of SHRA out of cloud above 5000 ft or out of a cloud of less than BKN below that. Because a FEW or SCT cloud does not impact the ceiling, therefore there is no obligation to include it, even though it would make sense. But SHRA can very well fall out of a CU which is less than BKN.
There is a 2nd group which implies the same with even stranger phenomena RA SHRA. It should be either or, depending on the clouds they expect.
Peter wrote:
What is the difference between cloud ceiling and cloud base?
Cloud base is the height of the lowest cloud, ceiling is the height of the main cloud base of BKN or OVC. So in a group of FEW015 SCT030 BKN040, the ceiling would be considered to be 4000 ft AGL in a TAF or METAR, while the base would be partly at 1500 ft, partly at 3000 ft and outside those at 4000 ft.
What is the difference between cloud ceiling and cloud base?
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.
I actually had a quick look at Chapter 3 Annex 5 and while it clearly says that changes in cloud ceiling (not base) should be reported if they pass through significant levels such as 1,500 ft as you write, I would interpret this as if there is a ceiling in the forecast in the first place, such as BKN020. Otherwise there is no change in the ceiling, but a new ceiling.
Peter wrote:
How is the SHRA in the bold section possible when no cloud is forecast for that period? Well, CAVOK is 5000ft AAL (or MSA if above 5000ft) but EGKK is almost at MSL.
There is no requirement to include a change group in the TAF unless any cloud base drops below 1500 ft. (Or even 1000 ft if the number of VFR flights is insignificant.)
TAF EGKK 050457Z 0506/0612 VRB03KT CAVOK
TEMPO 0506/0507 1200 BR
PROB30 0506/0507 BKN003
BECMG 0507/0510 15010KT
PROB40 TEMPO 0512/0518 7000 SHRA
BECMG 0518/0521 VRB03KT
BECMG 0521/0524 8000
TEMPO 0600/0606 3000 BR
PROB40 0600/0606 0300 FG BKN000
BECMG 0606/0608 9999
PROB40
TEMPO 0608/0612 5000 RA SHRA
How is the SHRA in the bold section possible when no cloud is forecast for that period? Well, CAVOK is 5000ft AAL (or MSA if above 5000ft) but EGKK is almost at MSL.
It implies rain showers from cloudbase above 5000ft. In the UK, this is extremely unlikely.
No idea how they handle it at EGKA but sure you can produce a TAF if sufficient information is available. In this day and age it is possible that they obtain what is necessary via automated stations nearby.
Occasionally it is done that TAFs can be produced by forecasters even for non metar airports.
And I have never ever heard of money involved in such stuff.
