How do they measure sky or cloud cover?

eetrojan

eetrojan

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I understand that the METAR reports CLR for 0/8, FEW for 1-2/8, SCT for 3-4/8, BKN for 5-7/8, and OVC for 8/8 - and for different cloud bases. I even learned a new term, "oktas," while looking around on the net.

However, I'm still curious about how exactly this is done visually, by a human observer. The geometry confuses me. Is it done by looking up and dividing the dome into 8 pie slices? A visual sweep over a worst-case arc? An estimate?

What exactly do the do? If relevant, what does an okta look like?

I can't seem to find a good explanation.

Thanks!
 
Man I love science!!

Thanks for sharing this and asking the question. I just learned something new :)
 
Science is fun, but that procedure has a huge number of problems with it.

The "real" process doesn't require entire solidly blocked segments, and does consider the whole sky from horizon to horizon.

When an observer says 4/8 of the sky is obscured, it doesn't necessarily mean one cloud that covers half the sky. It means half the sky is obscured; it can be distributed any way you like. From a grid of fair weather cumulus with intercloud spaces of roughly equal area distributed over the whole sky, to a linear cloud edge right over the observation site.

Another way is to make a single circular cloud right over the observation site, extending to 30 deg from the horizon.

Basically, count the fraction of the area on the sky that is obscured by the cloud.

There are a ton of science fair sites on the web, but I have yet to find one that leaves me with as many non-grey hairs as I started with.
 
Last edited:
MAKG1 said:
There are a ton of science fair sites on the web, but I have yet to find one that leaves me with as many non-grey hairs as I started with.
Click to expand...

:yeahthat:

Now, I have no better suggestion of how it's done, but that mirror approach doesn't really get you anywhere.
 
I am too lazy to look up how an AWOS works but that's as good an answer as you probably can find.

I suppose a human does something similar.

Cheers
 
register@teamandras.com said:
I understand that the METAR reports CLR for 0/8, FEW for 1-2/8, SCT for 3-4/8, BKN for 5-7/8, and OVC for 8/8 - and for different cloud bases. I even learned a new term, "oktas," while looking around on the net.

However, I'm still curious about how exactly this is done visually, by a human observer. The geometry confuses me. Is it done by looking up and dividing the dome into 8 pie slices? A visual sweep over a worst-case arc? An estimate?

What exactly do the do? If relevant, what does an okta look like?

I can't seem to find a good explanation.

Thanks!
Click to expand...

Wikipedia, under Sky Cover Measurements, is as good as it gets.

Bob Gardner
 
This thread reminds me of "smoke school", i.e. EPA-mandated training to judge how much smoke a stack is putting out...

home_main.jpg
 
alfadog said:
This thread reminds me of "smoke school", i.e. EPA-mandated training to judge how much smoke a stack is putting out...

home_main.jpg
Click to expand...

I though that was Old Faithful after OSHA Regs were applied.

Cheers
 
Thanks All! I have a bunch to read now.

However, first I'm going to have a beer because I just passed my stage 2 flight test!
 
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