A couple of hundred years ago clouds were not identified by name. Luke Howard, an English pharmacist and amateur naturalist, developed a system of classification (from Latin) for clouds in 1803. Howard categorized clouds into three major groups: cumulus (accumulate or piled up heaps and puffs), cirrus (fibrous and curly), and stratus (stretched out and layered). To further describe clouds, he combined those terms and used other descriptive words such as, alto (high), and nimbus (rain). Today, the International Cloud Classification is based on Howard's system.
There are three basic forms of clouds: cirrus, cumulus, and stratus. All clouds are either purely these forms, or a combination or modification of the basic forms. Since there is more water vapor at lower elevations, lower clouds appear denser than higher, thin clouds.