I'm not talking about the vertical difference in temps at altitude, such as at ground level versus at 1000 ft; I'm talking about where the land rises. So, that even at local ground level it's colder. I was watching a news report that indicated rain all across our province, except where the highlands are (Dundalk area, NE of London ON), and there it was snow. I get that air masses move across the land, and that the rise in ground causes the air mass to rise. Why does it cool? Does it really come down to adiabatic cooling? I know that, when air expands, it cools as per the Ideal Gas Laws. Is the expansion due to the the air rising a mere few hundred feet (from sea level) and - for no other reason than because it is less squished by the weight of air around and above it - it can expand? Is the highlighted part the primary mechanism?