Question: METAR Sky Condition

[tncmdream]

Good afternoon,

I cannot get this..although it looks so simple!



Have a look a this:
http://img487.imageshack.us/img487/5800/02bw2.jpg


2 examples:
http://img154.imageshack.us/img154/2136/01ld8.jpg




I would like to know how in these 2 examples, the cloud is calculated in /8 ?


In figure 2-1, i agree that at 2000FT we have FEW, i am adding 1/8 + 1/8 using the arrow <--- 1/8 --->, that makes 2/8.
However how is it possible for the one at 4000FT?
I mean it is 1/8 + 1/8, so 2/8 too..

Same question for figure 2-2.



Please advise,
Very good day

[AerocatS2A]

The higher layers are added to the lower layers to give cloud cover as viewed from the ground.

In example one:

2/8 at 2000' + 2/8 visible at 4000' is given as 4/8 at 4000'

My guess is that it accounts for the possibility that there is a further 2/8 at 4000' obscured by the lower cloud (as shown in the second example.)

I don't think it is done this way in Australia though as we are required to sum cloud amounts when assessing alternate minima. It seems that this is being done for you in the examples.

[punkalouver]

I would say that the layer at 4000 feet includes the coverage at 2000 feet. The observer doesn't know what the is hidden at 4000 feet behind the 2000 foot layer.

Imagine if 7/8 of the sky is cloudy at 2000 feet(a broken layer). You can see blue sky in that 1/8 of the rest of the sky. Then a small cloud at 4000 feet moves over that 1/8 of blue sky and completely covers it. You now have a 4000 foot overcast layer. Then the 2000 foot layer of cloud quickly moves off while the 4000 foot cloud remains unchanged in any way. A special is put out now as 4000 few(1/8 coverage).

So when you are descending into an airport You may find that a layer of cloud from your perspective is different than the observation on the ground although theoretically it should be more likely that the upper layers would have less sky coverage than reported. Of course clouds change and move and there is a time lag for a new report so perhaps there is more cloud as the picture has changed.

I have seen where a scattered layer(1/8 sky coverage) as observered from the ground was in fact a widespread layer of cloud at minimums on approach which shows that it is entirely possible to be on the missed approach after an entire approach in IMC when the airport is reporting just a few cloud.

Remember as well that higher clouds are usually just a guess at altitude.

[Spitoon]

Don't know where the examples come from but it's not how it would be done in the UK. In broad terms, the observer will report what he/she sees or has been able to see during the preceeding 10 minutes. So if there is solid 8/8 at 1000ft and another layer above at 3000ft the report would only include the layer at 1000ft. It's more complex when there are holes in layers and the observer may have to make educated guesses about the extent of higher layers seen through holes. Auto systems basically do the same but I don't think they include algorithms for educated guesses.

PS - "Remember as well that higher clouds are usually just a guess at altitude.".....when the CBR is broken the lower levels are too!!