Tukuk-OSA cloud is not steam and it is not about to “get heavy enough to drop”. It is a suspension of microscopic water or ice particles, typically 10–20 micrometres across, so light that rising air keeps them aloft indefinitely — a cumulus cloud can weigh close to a million tonnes yet float like it weighs nothing. For rain, each particle must grow roughly a hundred times in diameter, and clouds have two engines to do that.
In warm clouds, drops collide and merge — bigger falls through the cloud, sweeping up smaller ones, growing with every pickup until it finally outfalls the updraft. That is why tropical rain starts as fat, widely spaced drops. In cold clouds the trick is stranger: ice crystals and supercooled droplets can coexist, and because air holds less vapour over ice than over water, vapour migrates onto the ice, which fattens at the droplets’ expense until it falls — melting into rain on the way down. Most mid-latitude rain, and every snowflake, began with an ice crystal.
Even those tiny droplets usually needed help forming at all: water vapour cannot easily condense out of thin air onto nothing, so it condenses onto microscopic nuclei — dust, smoke, sea salt, even pollen and bacterial fragments. Clean air makes fewer, larger drops; polluted air makes more, tinier ones — which is why pollution can suppress rain while intensifying storms.