Tropospheres of Earth and Mars, Why does air temperature decrease with altitude? Options

[schaffman]

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

[qraal]

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

Hi Tom

Assuming you haven't found an answer - which an enterprising 10 year old probably would have by now - I can simply say that air loses heat as it gets higher because it gets tired on the way up. Though that sounds facetious I'm serious. Energy is lost as air rises under the effect of gravity - it has to climb against the drag of gravity, which means energy is expended.

[hendric]

I'd say it's also due to the greenhouse effect. Less air above you to help keep heat in. Think of a clear night vs a cloudy night. Those clouds do a great job of keeping the ground warm. But even on a clear, still night the air above you helps somewhat to keep you warmer, and at high altitudes there is less of it.

[Drkskywxlt]

The adiabatic lapse rate (-9.8K/km on Earth) is just acceleration due to gravity over the specific heat at constant pressure (cp). That's true for other planets as well. On Earth, you get an inversion (warming with height) in the stratosphere and the thermosphere because of enhanced radiative absorption from ozone and atomic oxygen, respectively. Mars doesn't have a true stratosphere and doesn't have any large region in the middle atmosphere that temperatures warm with height until you reach the thermosphere. Titan has a distinct troposphere, stratosphere, etc...

[schaffman]

Thanks everybody.