There is a constant exchange of water molecules between liquid water, or ice, and the air, as evaporation and condensation take place. Saturation vapour pressure (es) depicts a balance in the air between condensation and evaporation. es is greater in water droplets than in sheet water, and lower in impure water than in pure. Where es in an air parcel is greater than e (the ambient*vapour pressure) there will be net evaporation; where e is greater than es, there will be net condensation.
Subjects: Chemistry — Earth Sciences and Geography.