Cloud formation is based on simple physics. In general, clouds consist of liquid or solid ice water. They form when water from the ground or elsewhere evaporates into the air as invisible vapor.

The prefix “alto” (meaning “high”) helps distinguish mid-level clouds from their lower-level counterparts (such as altostratus vs stratus). NWS cloud chart. Click to expand to full-size.

A new result from the molecular gas survey in the Southern Pinwheel Galaxy M83 using the Atacama Large ...

A Met Office video explains seven rare cloud types - including noctilucent clouds as they’re sighted in the UK.

When cumulonimbus clouds grow high enough into the atmosphere, the temperature becomes cold enough for ice clouds, or cirrus, to form. Clouds made up entirely of ice are usually more transparent.