Why would water boil at less than 100 degrees on top of a mountain?

The boiling point of any liquid is the temperature at which its vapor pressure becomes equal to the atmospheric pressure. So if the atmospheric pressure is lower, it will take a lower temperature to make the vapor pressure equal to that of atmospheric pressure. At hill-stations, the air is generally thinner due to the altitude and the atmospheric pressure is also lower. Here, it requires less than 100oC temperature to reach the point where the vapor pressure of water reaches that of air. So, water boils below 100oC at hill stations.