Clouds are made of tiny droplets of liquid water or frozen ice crystals. Water vapor, although present in a cloud, is actually invisible. Whenever you see steam coming out of a tea-pot for instance, what you are actually watching is water vapor condensing into very tiny droplets of liquid water as the vapor comes in contact with the cool air outside the pot. In the case of clouds, you start with (invisible) water vapor mixed with air. Think of ordinary air around you on a humid summer day. If the concentration of water vapor is high enough (high humidity), the temperature is low enough (as in high altitudes), and there are microscopic particles to act as nucleation centers (such as dust) then, part of the vapor condenses into tiny (about 0.01mm in diameter) droplets. These droplets can remain in a liquid phase but, if the temperature is even lower, they will freeze and become ice crystals. For pictures of clouds, including ice-crystal clouds, have a look at click_here
Please focus on the High Clouds section near the top of that page. Hope this helps...
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