I recently finished reading "My First Summer in the Sierra", by John Muir, specifically the edition published by the Standard Ebooks project. I highly recommended it to fans of nature writing! I found it something to enjoy in many relatively short installments. The sheer intensity of Muir's exuberance in nature, and his poetic rendering of it, is such that you can't really just sit down and plough through it. Your mind kind of by necessity desensitises to it a bit if you try.
Standard Ebooks: "Free and liberated ebooks, carefully produced for the true book lover"
One thing has been bugging me. Muir regularly begins his description of a day by reporting on the cloud cover. This is expressed as some unitless quantity which is a completely mystery. The only system I am familiar with for measuring cloud cover is in eights of the sky covered, also known as "okta". These are usually represented numerically with integers between 0 (sky completely clear) and 8 (sky completely cloudy). One could of course convert them to a proportion, but then we'd be talking about rather cumbersome numbers like 0.375. The majority of Muir's reports are something like 0.05, 0.10 or 0.15, which at first suggests a system based on the 20ths of the sky covered, but I noted one case of "only about 0.01" (clarified as faint, silky cirrus wisps, scarcely visible) and another of "only about 0.08", which suggests something yet finer-grained. How on Earth, though, does one measure proportion of sky covered by cloud down to a resolution of 0.01 in 1869, not in a lab but while camping and hiking through forest and mountain landscapes with a crew of shepherds? Does this scale not in fact top out at 1.0?
The Wikipedia articles for "Cloud cover" and "Okta" make no mention of alternative systems, or predecessor systems. I have tried searching the web for hints but, well, we all know how terribly that works these days. It's especially frustrating that it seems like no matter what other search terms I include, it never seems to take terribly long before results relating to "the other cloud" start filtering in. Some clever so-and-so has even decided to use the name "okta" for a piece of software related to some cloud nonsense or another, which does not help. I've basically given up on finding an answer to this myself, and hope somebody else reading this, as improbable as it would be, has some clue.
English Wikipedia article - Cloud cover
English Wikipedia article - Okta
This small research effort was not a total loss. I learned of the existence of a very cool instrument for recording the total duration of bright sunshine per day, the Campbell-Stokes sunshine recorder. It is not much more than a large spherical lens which focuses sunlight from anywhere in the sky onto a thin piece of card. When the sun is bright enough, the focussed light burns a hole in the card, in the manner one might have used a magnifying glass to burn paper or even ants as a child. As the sun moves across the sky, so too does the point on the card where its light is focussed, burning a straight-line track. Intermittent periods of heavy cloud cover correspond to breaks in this track. The card is printed with gradations indicating the time of day, so one can translate the presence or absence of burning to when in the day the sun was up and bright and when it wasn't. It's delightfully simple and low-tech, requiring no external source of power and not even having any moving parts! Allegedly they are still in widespread use today. I'm a little sceptical of that claim, but I kind of hope it's true.