Clouds develop when moist air cools to its dew point by rising to a higher altitude or by moving over a cooler surface. Water vapor in the air then condenses in liquid or frozen form around minute particles such as pollen or dust. The shapes and altitudes of clouds, as well as the sequences in which they develop, help forecast the weather.

In the early nineteenth century, Englishman Luke Howard, a chemist by trade and meteorologist by avocation, created a system for classifying clouds using Latin names. He described the three most common shapes as cirrus (curl of hair), stratus (layer), and cumulus (heap); he also defined four compound cloud forms that derive from the three primary shapes. Later, scientists added terms such as humilis (small) and incus (anvil) to designate other cloud properties. Arranged according to altitude, nine of the ten basic cloud genera are pictured on this stamp pane. Information about each cloud image is included on the back of each stamp.