* Decreasing Air Pressure: As you ascend a mountain, the air pressure decreases significantly. Air pressure is directly related to the density of air molecules. With fewer air molecules, there's less air to trap heat.

* Adiabatic Cooling: As air rises, it expands because the pressure decreases. This expansion causes the air to cool. This cooling process is called adiabatic cooling. For every 1,000 meters you climb, the temperature drops roughly 6.5 degrees Celsius (11.7 degrees Fahrenheit).

* Less Atmosphere to Trap Heat: The higher you go, the less atmosphere there is above you. This means there are fewer layers of air to absorb and reflect heat from the sun back down to the surface.

* Radiation Loss: At higher altitudes, the air is thinner and drier. This allows heat to radiate away from the surface more easily, leading to colder temperatures.

In summary: The combination of lower air pressure, adiabatic cooling, less atmosphere to trap heat, and increased radiation loss all contribute to the colder temperatures found at mountain peaks.