To form clouds and rain, water vapor (water that has evaporated into the air) needs both cool temperatures and a surface on which to condense (turn back into a liquid).
As warm, moist air rises outdoors, it carries water vapor high into the atmosphere where there is less air pressure. The air expands, causing it to cool. Its water vapor condenses on atmospheric dust and may form a cloud.
Indoors, warm, moist air can’t rise high because of ceilings. It can cool by meeting up with cold surfaces like windows. But any drops that form on airborne dust don’t get large enough to fall as rain.
Art by: Susan Batori