Mountain regions are cooler than plains because of the following reasons:
1. Air becomes thinner with the increase in altitude. The thin air cannot absorb and retain much of the heat because air on the mountains is less dense compared to the air on the plains.
2. Mountains receive plenty of rainfall because of the rapid condensation of the clouds owing to the cold conditions at high altitudes.
3. Snow forms on the mountains because of the low temperatures and keeps the air cooler in the mountain regions for most part of the year.