Why are temperatures most often cooler in the mountains than in the valleys?

Temperatures are cooler in the mountains than in the valleys primarily due to the decrease in atmospheric pressure and density with elevation. As altitude increases, the air becomes thinner and less able to hold heat, leading to lower temperatures. Additionally, the process of adiabatic cooling occurs when air rises, expands, and cools, further contributing to the cooler mountain climate compared to the warmer, denser air in the valleys.