A cloud is water vapor in the atmosphere (sky) that has condensed into very small water droplets or ice crystals that appear in visible shapes or formations above the ground.
Water on the Earth evaporates (turns into an invisible gas) and rises up into the sky. Higher up where the air is colder, the water condenses: it changes from a gas to drops of water or crystals of ice. We see these drops of water as clouds.
Cumulus clouds are often described as “puffy”, “cotton-like” or “fluffy” in appearance, and have flat bases. Cumulus clouds are low-level clouds. Cumulus clouds may appear by themselves, in lines, or in clusters.
Stratus clouds are low-altitude gray clouds that make a flat base. The name comes from the Latin word stratus, which means to “stretch” or “extend.” You can see stratus clouds as thick cloud blankets near the sea. They are sometimes called “high fogs”. Light rain and drizzle often fall from stratus clouds.
Cumulonimbus, from the Latin cumulus (“heap”) and nimbus (“rainstorm”, “storm cloud”), is a dense towering vertical cloud associated with thunderstorms.