The vapour that escapes into the air whether by evaporation or sublimation have sufficient energy to break away from the molecular bond of the water. The vapour molecules are in constant motion exerting a force on the container or the air around. This force is termed vapour pressure. The atmosphere contain molecules that are in constant motion. They exert a downward force on a liquid’s surface. The higher the air pressure, the harder it is for the liquid to evaporate. Therefore, the boiling point of a solvent or liquid is affected by the atmospheric pressure and boiling point is raised.
When the atmospheric pressure is reduced, the water molecules on the surface of the water need less energy to bounce upwards to escape into the atmosphere. As elevation increases, atmospheric pressure and boiling point decrease because air is less dense at higher altitudes. If the atmospheric pressure is lower, the vapour pressure of the liquid needs to be lower to reach boiling point.