Private pilot* here. One thing that isn't obvious from ground level is that the bottoms of clouds are relatively flat - in spite of the tops and sides being well, irregular as clouds.

*actually ex-pilot, gave it up when I turned 65

As I understand it, that's because the water vapor starts to nucleate (form tiny droplets) on bits of dust as the atmosphere 'supersaturates' (has more than 100% humidity in it). As air temperature drops, it's 'carrying capacity' to hold water as vapor drops - and as it rises it gets cooler. So when it rises high enough to get cool enough to have more water vapor than it can carry it starts turning into cloud - and that usually starts at just *one* height above ground - so flat(ish) bottoms.