Clouds are formed when wind induces water evaporation. Wind also stirs sea mist as well as dust particles, and cosmic ray-ionized gases. These act as nuclei to collect water vapor of the atmosphere into clouds. This is expressed in the Qur’ân.
§ Allâh is He Who sends the winds, so that they raise clouds. [30/48]
§ And it is Allâh Who sends the winds, so that they raise up the clouds. [35/9]
It is as though wind inoculates or “fertilizes” the clouds with these various particles. This very same description is used in the verse:
§ And We send the winds fertilizing, then cause the water (rain) to descend from the sky. [15/22].
Further, the wind drives the clouds here and there. Oppositely charged clouds are “combined” together into “heaps” or piles, as in the Qur’anic verse:
§ See you not that Allâh drives the clouds gently, then joins them together, then makes them into a heap of layers?, and you see the rain comes forth from between them, and He sends down from the sky from hail (like) mountains (or there are in the heaven mountains of hail from where He sends down hail). [24/43].
These combinations result in “heavier” and more dense clouds, that are more amenable to rain fall. This process is accompanied with intense electric discharges, manifested as lightning and thunder, as in the following verse;
§ It is He Who shows you the lightning, as a fear (for travelers) and as a hope (for those who wait for rain). And it is He Who brings up (or originates) the clouds, heavy (with water). [13/12-13].
Spreading of clouds in the sky can take one of two modes: either horizontal spreading with little height; or vertical buildup of layers of ice (cumulonimbus). The latter take mountain-like shapes, an observation only recently made during air flights. Qur’ân differentiates clearly between these two modes, using different expressions:
§ Allâh is He Who sends the winds, so that they raise clouds, and “spreads” them along the sky as He wills, and then break them into fragments, until you see rain-drops come forth from their midst [30/48]
§ See you not that Allâh drives the clouds gently, then joins them together, then makes them into a heap of layers?, and you see the rain comes forth from between them, and He sends down from the sky hail (like) “mountains” (or there are in the heaven mountains of hail from where He sends down hail). [24/43]
It is the latter cumulonimbus clouds that normally give hail stone, as well as thunder and lightning. This is the context of the latter verse above (24/43), which also indicates that rain comes from “within” the clouds, and not from the bottom surfaces, as laymen and our early ancestors could have imagined.