Vapor pressure is the pressure exerted by the vapor of a liquid when in equilibrium with the liquid. At a given temperature, the vapor pressure of a liquid increases as the atmospheric pressure increases. This is because the higher the atmospheric pressure, the more molecules of the liquid are forced into the vapor phase in order to reach equilibrium.
At high altitudes, the atmospheric pressure is lower than it is at sea level. This means that the vapor pressure of a liquid is lower at high altitudes than it is at sea level. As a result, a liquid will boil at a lower temperature at high altitudes than it will at sea level.
