Clouds seem to form in units. I.e. a cloud here, and a cloud there. Each cloud looks (at least from the ground) to have a very definite surface. I know that when you get up close, it is more fuzzy and gradual, but it’s still far more sudden than I would expect.
I know about humidity and dew points and particulate matter and pressure … all the things that lead to cloud formation. But once a cloud begins to form, why does it “grow” like it does? Why wouldn’t it just sort-of appear all at once? And why wouldn’t it have a much more gradual change from clear air to cloud?
Is there something about the presence of cloud droplets that makes it more likely for further cloud formation to happen near-by?
If you look at a waterfall from the side you can see where ‘water’ stops and ‘spray’ starts. I wonder if it’s not the same with clouds? The difference being that we’re so far away from clouds it’s more difficult to make out the ‘line’ of saturation.
Some clouds do appear all at once, and produce “pop-up” showers, which meteorologists call instability or convection showers.
You have to look at the meteorological conditions that cause clouds and rain. A Low pressure will move W to E here in the prevailing westerlies, along with a cold front. The center of the Low will move along the front as the front advances from W to E, ususally NW to SE. As it does so, it sends ahead of it clouds, which far enough ahead are very thin, called cirrus clouds (they are sometimes called “mares’ tails.” These are high, thin clouds that foretell the approaching bad weather. Call a point ahead of the front Point X. Point X will see the approach of cirrus clouds, then the clouds will settle in and form a layer of clouds. These are called stratus clouds. And when the rains and snows come, they will thicken appreciably. These are called nimbus clouds. These varying formations are caused by the meteorological conditions which you alluded to in your 3d paragraph: increasing moisture, warmer air rising, etc.