Craig.Everhart%CMU-CS-A@sri-unix.UUCP (01/18/84)
Water vapor condenses out of the air to make water droplets. Water vapor is transparent to visible light; the surfaces of the droplets of water are not. We see water droplets as fog or clouds. If the condensation is great enough, the water falls as rain. If the condensation happens at sub-freezing temperatures, and is great enough, it falls as snow. If the condensation isn't that great, then the droplets (or ice particles) can remain suspended in the air--like the chocolate particles in cocoa. The solubility of water in air varies with the temperature and pressure of the air. Moist air rising from the surface of the earth cools off and becomes less dense; one or both of these factors reduces the solubility of water in that air to the point where it condenses out. When this happens, clouds are formed. (This formulation also suggests why it is that the bottoms of clouds seem flat.) Moist air can cool off without rising, too. If a body of moist air cools off (overnight, say) while it continues to sit on the ground, we get fog. I don't know why this stuff should be a mystery. Even though I don't know the precise details of the solubility of water in air, I know that temperature and pressure will affect it, and that the temperature and pressure of the air in the atmosphere varies from place to place. The consequences (of having clouds at all kinds of altitudes) are not surprising. (A third variable is how much water vapor there is in a given body of air. Even when the solubility of water in air goes down, if there is so little water present in the air that it can all remain in solution, it may very well do so.) Craig Everhart