Chemistry · EQUILIBRIUM

At high altitudes, liquid water boils at a lower temperature than at sea level. What is the fundamental thermodynamic/equilibrium reason for this observation?

1. The heat of vaporisation of water decreases substantially at higher altitudes.
2. The rate of evaporation of water increases dramatically at high altitudes due to lower air resistance.
3. The normal boiling point of water is an intensive property and changes with altitude due to gravitational variation.
4. The vapour pressure of water required to equal the external atmospheric pressure is achieved at a lower temperature because atmospheric pressure is lower.