But tilt the barely stable pile just a little more, and a sudden surface avalanche pours downslope, leaving the deeper-lying seeds entirely undisturbed. Pour water into a tall cylinder, and the pressure at the bottom will steadily increase with water level. For granules, the bottom pressure soon reaches a limit and grows little more under greater depth. Grains feel friction and send forces sideways to share the weight between bottom and walls. The real temperature of the grain material is not directly relevant, for the thermal motions of atoms within the grains do not affect the motion of a grain as a whole.
Friction surely does, and internal energy losses dominate the repeated collisions between grains.
The lossy grain motion diffuses swiftly into abrasion, heat, even sound, and the energy of motion is all but gone after a few dozen collisions. When an atom gains or loses energy of motion, it has merely passed it to a partner. For real atoms rebound perfectly until a quantum jump occurs, an event rare at modest temperatures. In the time a moving seed requires for 1,000 collisions with neighbors or walls, an air molecule will collide and rebound a trillion times. Big, slow grains have no time to search out all the opportunities open in the swifter world of atoms.