Astronomy Images of astronomical nebulae look startlingly like clouds on the sky on the earth, even though they are vastly different in size and composition. Interstellar clouds are composed mostly of hydrogen, whereas terrestrial clouds are formed from water droplets or ice crystals.
Terrestrial clouds come in diverse forms.
Take common cumulus clouds as an example. These are the fluffy, white, dome-shaped clouds that have flat bases and wispy tops. This restless process occurs because cumulus clouds are created by convective motions of moist air driven by heat from the sun-warmed earth. As a cell of warm air rises, the surrounding atmospheric pressure decreases, allowing it to expand. The excess vapor condenses, forming water droplets or, if it is cold enough, ice crystals.