Clouds are classified and grouped into "Low", "Middle", and "High" clouds, referring to the altitudes they occur at. Low clouds include cumulus, nimbostratus, stratus, and stratocumulus; middle clouds include altocumulus and altostratus; and high clouds include cirrus, cirrocumulus, cumulonimbus and cirrostratus.
- Compared to cool air, warm air is less dense and is capable of holding more moisture in the form of water vapor. As a result of its low density, warm air low in the atmosphere tends to rise, expand, and cool. Every 1000 feet, it cools 5.5 Fahrenheit (10°C every 1000 m). Since this cooled air is no longer able to hold as much moisture, its moisture begins to condense into water droplets and form clouds. The opposite effect occurs in the atmosphere when air descends--it warms and dries. 
Clouds and aerosols
"Cosmic rays are charged particles that bombard the Earth's atmosphere from outer space. Studies suggest that cosmic rays may influence the amount of cloud cover through the formation of new aerosols (tiny particles suspended in the air that seed cloud droplets)." 
- ↑ A cloud is a large collection of very tiny droplets of water or ice crystals. The droplets are so small and light that they can float in the air. Crystal Wicker, weekend meteorologist for WRTV-TV in Indianapolis, Indiana.
- ↑ CORRECTION: Clouds are made up of liquid water droplets or ice crystals which are suspended in the air by random turbulent fluctuations within the cloudy air. They do not actually float, as the droplets and crystals are not lighter than air. Cloud droplet sedimentation (or gravitational settling) is the process of cloud droplets falling, this process is impeded by small scale turbulent updrafts, so the net effects are that clouds are suspended. When the droplets grow large enough (through collision and coalescence), they fall freely out of the cloud, producing rain drops and thus precipitation. See, for example, A Short Course in Cloud Physics by Rogers & Yau
- ↑ http://www.cotf.edu/ete/modules/elnino/cratmosphere.html
- ↑ http://www.pparc.ac.uk/Nw/cloud.asp