Explanation:

As the temperature of a liquid is increased the vapor pressure (i.e. pressure exerted by vapor when it is in phase equilibrium with a liquid) of that liquid also increases thus a point reaches at which liquid molecules have sufficient energy to leave free surface of liquid in form of vapor. This process induced by external heat energy is called ‘Boiling’. The temperature at which the vapor pressure of a liquid becomes equal to the surrounding pressure or a liquid starts boiling is called boiling temperature. It is dependent on the atmospheric/surrounding pressure.

The boiling temperature increases with increase in atmospheric pressure & vice versa is also true. Boiling temperature decreases with increase in altitude because of decrease in atmospheric/surrounding pressure.