Why it is cooler at the top of a mountain than at sea level?

Why is it cooler at the top of mountains?

Why a liquid boils at a lower tempetature at the top of a mountain than at sea level ?

On the top of a mountain water boils at

At the top of the mountain the temperature is -13^(@)C and the barometer reads 68 cm of Hg. At the bottom the temperature is 27^(@)C and the barometer reads 70 cm of Hg. What is the ratio of the densities of the air at the mountain to that the sea level?

On the top of a mountain, water boils at