What causes the lapse rate in the tropospheres of Earth, Mars, and Venus?
I used to think that it was because thinner air has a lower heat capacity and absorded less radiation (both incoming short-wave from the Sun and outgoing infrared). But I've also read that air at high altitudes is cooler because it is farther from the ground (i.e, it is less warmed by the infrared-emitting surface). This makes sense if you're in a balloon or airplane, but what about on a wide, high plateau where you're on the ground. Isn't the air is still cooler there than at sea level?
Without going into dry vs. wet lapse rates or effects of convection, is there a simple answer to this question that a fifth grader could understand?
Tom