Lets divide the atmosphere into spheres of equal size. The molecules in warm air bounce off each other more, meaning that each warm sphere will have fewer molecules in it and therefore less mass. Likewise, cold air molecules bounce off each other less, so each cold sphere will have more air molecules in it, and therefore have more mass. Spheres with more mass weigh more than spheres with less mass, so the former descend and the latter rise relative to each other.

As warm spheres rise, two things happen:

1. pressure falls because there are fewer and fewer molecules above a sphere pressing down on the whole atmosphere. As pressure goes down, energetic molecules spread out and bounce into each other less often, which reduces their temperature; and

2. air moves away from its main heat source: Earth's surface. More of its energy simply radiates away into space.

So rising air steadily cools, but continues to rise so long as it remains slightly warmer than adjacent air masses.