Here's why:

* Atmospheric Pressure: Air pressure decreases as you go higher. This means there are fewer air molecules pressing down on you.

* Heat Retention: Air molecules are what trap heat from the sun. With fewer molecules at higher altitudes, less heat is retained.

* Adiabatic Cooling: As air rises, it expands. This expansion causes the air molecules to spread out, decreasing their kinetic energy (the energy of motion). This decrease in kinetic energy translates to a drop in temperature.

This is why mountains are often snow-capped, even in areas with warm climates.