- the ratio of the bounding rectangle is indeed between about 1.60 and 1.64 depending on how you fudge the pixels. but only for the inner cloud shape, including the embossed highlights, not for any of the other four possible measuring points.
- the ratio of the two small circles might be 76:47 if you again fudge the pixels just right, but then it's 1.617, so that's really close.
- the ratio of the two larger circles on the right is way off. 106:71 is a ratio of 1.49 and that's not going to come anywhere near the golden ratio no matter how much you fudge it.
Hey if anyone got a much higher resolution version of this icon I could measure the ratios to within some precision, that might be nice.
Additionally the ratio between the biggest and the smallest circle is almost exactly 4:9. I bet that's really significant too. Because 9 - 4 = 5, and as the great Malaclypse the Younger already stated in his magnum opus Principia Discordia: "All things happen in fives, or are divisible by or are multiples of five, or are somehow directly or indirectly appropriate to 5. The Law of Fives is never wrong."
Quantum Eris Demonstrantum
Funny thing about the Golden Ratio, if you look for it, you'll find it everywhere.
But let's say that's what I meant.
Apple's MobileMe was announced on June 6, 2008. From the start, it used the same cloud icon. 
According to the Pictos web site (and the Wayback Machine), the Pictos collection came into being in 2010. 
Edit: I thought of one possible explanation. The author, a designer at Microsoft, explains his reason for writing the article is because people accused him of copying Apple's cloud icon for use on ASP.net's website. This made him search for earlier examples than Apple's, to somehow justify using the same cloud shape that so many lazy designers do now.
Having the same general shape and layout is one thing but being exactly, pixel-precise the same is quite another.
"we used an icon from the Pictos collection. I have an email from March of 2010 where we selected that icon, in fact."
"we updated the old site's cloud icon"
"the icon isn't from Apple, it's straight from Pictos 1. I know, because we bought it from them for our site."
"it seems there is only one cloud icon in the universe and it's four circles with a flat base. I like it."
Nevertheless, though the author talks about the Pictos 1 icon set being around for years, he offers no proof for that. The Pictos website was made in 2010, years after Apple's MobileMe.
Also, a simple Google image search of [cloud logo] gives many different varieties of cloud. Some have bumpy bases; some have more or less than three circles; etc.
Counter-argument to this:
tl;dr: It's everywhere only if you cherry-pick what you measure and occasionally squint.
I think the more interesting topic is why asymmetric, flat-bottomed, 4-bulged, 3-radiused, golden-aspect-ratio'ed is so popular in the icon format. Pretty sure it's due to the need to look good bounded in a small square – balanced but not perfectly (unnaturally) so, and simplified to minimal details but not cartoonish.
Apple released the first iPhone SDK beta on March 6, 2008. 
Apple announced MobileMe on June 9, 2008. Both MobileMe and the iPhone App Store launched on the same day, July 11, 2008. 
So there was a 3 month period in which some app developer could've independently come up with a cloud themed app icon identical to the one used in MobileMe. I doubt that happened, though.
Except that BBC cloud icon looks nothing like the iCloud icon.
when you're presented with some 48 figures, you simply overload your brain and it fails to evaluate aesthetics of each rectangle, sort and pick the highest score. so in my opinion that experiment is flawed.
you brain can only operate on a limited set of objects especially when evaluating and comparing qualities. (7+-3 or thereabouts). pick any book about brain/psychology - it's there.
I'll check the PDF, thanks. Also see:
People do seem to like ratios of small integers, though. Which is why the supposed aesthetic elegance of 1.618 ((1+sqrt(5))/2) is most probably within statistical significance of 1.5 (3/2) or 1.666 (5/3), it's impossible to test whether people don't just enjoy ratios of small integers instead of the irrational number Phi.
The flat bottoms however, hmm…
Well, wavephorm has a good answer to that elsewhere in the discussion: Because clouds are flat on the bottom and bumpy on top. ;)
You could do a more stretched or jagged looking cloud, but it could easily end up implying 'danger' or 'bad-weather'.
The rounded happy-cloud does seem the obvious first choice.
Deviate too much from the agreed upon shape, and you're likely to get a lot of "What's that?"
According to studies done on the matter, nobody has demonstrated said preference: