Why is air progressively cooler at higher elevations?

(The following explanation seems plausible:

The sun's heat energy absorbed during daytime by large bodies of water around or within lands gets slowly released--radiated--to the air mass close to ground and water surface. There is slow air convection both above ground and bodies of water, but the slowness of convection and continued absorption of sun's heat every day by bodies of water must
maintain air near the land surface warmest all the time and air at higher elevations progressively cooler.)