1. Altitude: As you go higher in altitude, the air becomes thinner. This means there are fewer air molecules to absorb and retain heat from the sun. The thinner air is less efficient at holding onto heat, causing temperatures to drop. This is known as the adiabatic lapse rate.

2. Solar Radiation: Mountains receive less direct sunlight compared to lowlands. Sunlight hits the slopes at an angle, spreading the energy over a larger area. This reduces the amount of heat absorbed by the mountain surface.

3. Air Circulation: Mountains can disrupt air circulation patterns. Cold air from higher altitudes can easily flow down slopes, creating cooler microclimates. Additionally, mountain ranges often block warmer air masses from reaching areas on their leeward sides.

4. Snow and Ice: Mountainous regions are often covered in snow and ice, which reflect sunlight and have a low heat capacity. This further contributes to colder temperatures.

5. Reduced Plant Cover: Higher altitudes have less vegetation, which would otherwise absorb heat from the sun. The lack of trees and shrubs allows for greater heat loss through evaporation and wind.

In summary: The combination of thinner air, less direct sunlight, cooler air circulation, snow and ice cover, and reduced plant cover all contribute to the colder temperatures found in mountainous regions compared to lowlands.