Cumulus clouds are those fluffy confections floating through the sky that are most commonly thought of as clouds. The ones every young child at daycare or kindergarten includes with the sun in any picture showing the daytime sky. The name comes from Latin and means heap or pile. They take on all manner of forms and cotton-candy sculptures to invigorate humanity's imagination, invoked by the brain's "pattern-recognition wetware", and form fantastical landscapes below travellers through the tiny windows of that Boeing 747 crossing the skies to distant lands.
Clouds are visible formations of liquid or solid water droplets held up against gravity by air pressure. They may be composed of micro-droplets of water, larger droplets formed around particulates in the air and/or ice crystals of varying size. Clouds are classified based on their composition, shape, altitude above the Earth's surface and their effect on the weather of the geographic locality below them.
Cumulus clouds are puffy masses of cloud, seemingly solid, that gift human imaginations with three dimensional pictures in the sky. That face of someone known in the past or some astounding creature from mythology, but sometimes it may be a dark and demonic hooded figure hovering above, threatening heavy rain and lightning. All imagination of course, but it is the cumulus cloud that most often draws people's eyes to the sky, distracting from day to day worries, if only for a moment.
From light fluffy confections to dark hovering thunderheads, cumulus clouds can change their aspect and temperament in stunningly short intervals. They form over warm ground or seas in moist air; as the air rises it cools, which reduces its capacity to hold water vapor. The excess water vapor liquefies into micro-droplets of liquid water, becoming visible as cloud. The base of a cumulus cloud tends to be fairly flat and relatively uniform with the other cumulus clouds forming in a given area because the air pressure in such regions is typically also uniform, the density at base level and the height the cumulus cloud can tower to, is dependent on the severity of the locality's temperature gradient. If the temperature above the Earth's surface drops away slowly with altitude, you get fluffy white cumulus clouds. If the decline in temperature is more rapid, you tend to get dark bases supporting towering thunderheads.
Cumulus clouds are an important indicator to the space program. Because cumulus clouds with tops above the altitude of freezing level can become thunderheads very rapidly, NASA's space shuttle launch programs used to operate under the cumulus cloud rule. No launch was permitted if the trajectory of the shuttle would pass through or close to cumulus clouds defined on the basis of the clouds height and whether or not it was producing rain.