On a hike, we told our daughters snow is found higher since the air is colder at altitude. Our 9 year old pointed out-doesn't cold air sink?
Smart girl! HOWEVER...it IS colder at altitude because temperature ultimately measures the motion of air molecules. Warm air means lots of colliding air molecules. It's what we call high kinetic energy. Cold air, on the other hand, means fewer colliding air molecules. At higher altitudes, such as the area you were hiking...the air is less dense...and there are fewer molecules available to collide. That's why temperatures are colder...usually. (The exception would be if a temperature inversion was present). Finally, cold air does sink. Your 9 year old is correct about that. But as that cold air sinks, it compresses or squeezes. The increasing pressure leads to increasing temperature (more colliding air molecules) and that's what keeps the temperatures warmer close to the ground. Some valleys can trap cold air overnight, leading to very cold temperatures. But that's an exception. By the way, you can demonstrate the effect we spoke of to your daughter by showing her what's called the 'ideal gas law'.