This page will make more sense to you if you have come from the page on heat transfer. Cold air sinks. The smoke that comes from a cigarette rises because it is being carried by hot air.
Warm air can hold water.
Air that is growing cooler releases water. Here warm air in a room has come in contact with a glass of ice water. The moisture in the chilled air has condensed out, leaving drops of water on the outside of the glass. Now let's see how this affects the rainfall on a planet. As the sun rises, the land and water on a planet begins to warm up.