Roger Worden wrote:
I forget where I read this... but it seems to apply. An author pointed out
that it is very hard to estimate the size (therefore the distance) of
typical clouds because their borders are fractal. (A fractal is a line or
shape that has a fractional dimension. Wikipedia defines it as 'a shape that
is recursively constructed or self-similar, that is, a shape that appears
similar at all scales of magnification and is therefore often referred to as
"infinitely complex".') The little swirls of the cloud are shaped exactly
like the big parts. That means that you can't tell objectively whether the
curlicues that make up the cloud are "small and close" or "big and far".
When you get closer to a cloud, the details look just like the broader view.

So unless you have an object near the cloud to compare it to, such as a
mountain or an aircraft, it's tough to objectively estimate the cloud size
and therefore its height. Watch a typical Cu, estimate how big it is... then
be amazed at how big it REALLY is (and therefore how high or far it must be)
when an airliner flies in front of it. If there are clouds near the
mountains, sometimes you can estimate their size and altitude and then
assume the rest of the visible clouds are at a similar altitude.

A cloud's motion can be another clue... but only if you know something about
the wind. Is that cloud "big and far and moving fast" or "small and close
and moving slow" ?

The white-to-gray gradient can also be a clue if you can see the side of a
cloud.

Not sure if this helps... but it explains why it's hard!

Roger


Interesting. Our premise was actually slightly different; although it
is "hard", we seem to be able to do a remarkably good job of estimating
cloud base visually even without reference objects such as hills,
airliners, etc. That said, I tried this yesterday (a day with
extensive OD here in NJ) and failed pretty miserably. I was guessing
4,000 feet or so, until a 777 came by well below cloudbase. Where I
am, that means about 6,000 minumum...

I'm still of the opinion that people can generally guesstimate +/-
1,000 feet AGL at least in the eastern US where bases typically only
vary from 3,000 to 8,000 AGL. I still can't explain why.

P3