Reading the clouds?

[Kaibutsu]

Hi everybody,

Need some help please.

On some bomb mission reports, crews mention clouds being 2/10, 5/10 10/10 and tops 4/1000 6/1000.

(example) "Bombing crews report 7 to 10/10 clouds over target with tops 6/1000 above, visibility was very good."

How do I read the numbers to make sense of the cloud cover?

TIA
Andrew

 

[Airframes]

Quite simply, the fractions are describing the amount of cloud in a given area visible to the observer. Imagine you are in a box, say five miles by five miles. If the cloud is below you, and filling half the capacity of that box, then, of course half = 5 tenths, so that's 5/10, or 50% of the area. If it's 8/10, that's eight tenths, so 80% of the visible area is covered by cloud, and so on.
The 'tops' figures mean the top of the cloud level to the base of the cloud, although some organisations use this as the height of the top of the cloud above ground level.
Therefore, 6/1000 would mean cloud tops at 6,000 feet.
The 'tenths' system has now been replaced by 'Octas', so a description of cloud cover today might read, for example, 7 octas, 10,000 feet. This means approximately 75% cloud cover, with the cloud tops at 10,000 feet.

 

[mikewint]

Terry, probaly to hard just to say "percent". But my understanding is that okta refers to "8" thus 7 octas means 7/8th of the sky covered or approx 88% be I wrong?
In aviation weater forecasts: SKC = Sky clear; FEW = 1 to 2 Oktas; SCT = 3 to 4 Oktas; BKN = 5 to 7 Oktas; OVC = 8 Oktas. NSC CAVOK = Nil significant cloud.

 

[Airframes]

Mike, you're quite correct. I used the term "approximately 75%" to avoid confusion with the octa descriptions versus the 'tenths' system.
The 'tenths' were replaced, apart from 'metrication', mainly due to the difficulty, in some instances, of an individual deciding what is, for example, 6/10ths or 7/10ths, where to be so relatively precise could actually be difficult.
With, for example, '7 Octas', this is obviously more than half, but less than the whole, so although 88% is the specific answer (which would need angles wings and a very large tape measure to achieve!), then 'approximately 75%', or three quarters of the sky covered by cloud, is good enough to provide a visualisation.
There is a slight difference between 'NSC' and 'CAVOK', especially in Europe, where the former may mean very light, small clouds, in a very wide spread pattern, in an otherwise clear sky, and the latter may mean no cloud, no haze, and visibility OK. I have forgotten what the latter looks like!!

 

[Kaibutsu]

Thanks for the help guys!

 

[Airframes]

You're welcome.

 

[Capt. Vick]

Terry the Tome!

 

[mikewint]

You betcha...that he is