An increase in atmospheric pressure raises the boiling point of a liquid by raising the vapor pressure of the water above the liquid. This increases the amount of thermal energy needed to increase the vapor pressure of the water to match, raising the boiling point. read more
Because the atmospheric pressure is lower, the vapour pressure of the liquid needs to be lower to reach boiling point. Therefore, less heat is required to make the vapour pressure equal to the atmospheric pressure. The boiling point is lower at higher altitude. Here's a video that demonstrates the effect of atmospheric pressure on boiling point. read more