Lecture 3: Water in the atmosphere 2

Question 1

STAGE 1: EVAPORATION EXCEEDS CONDENSATION

Answer 1

• At the surface, water molecules with sufficient speed (kinetic energy) will break away from the surface and evaporate into the air.
• With time the amount of water vapour in the air increases as the liquid water evaporates.•

Condensation (return of water molecules from the vapour phase to the liquid phase) also will occur (this is not shown in the figure above), however the rate of condensation is smaller than the rate of evaporation.

Question 2

STAGE 2: DYNAMIC EQUILIBRIUM - RATES OF EVAPORATION AND CONDENSATION ARE EQUAL

Answer 2

• The rate of condensation (return of vapour to liquid water) depends on the amount of water vapour in the air.
• Once the air above the water surface is saturated with water vapour, for every water molecule that evaporates one water molecule condenses.
• At this stage the amount of water vapour above the surface will not further increase. A dynamic equilibrium has been established between the evaporation and condensation processes.
• If water vapour molecules above the surface are transported away by wind, the saturation point will not be reached and the evaporation rate will continue to exceed condensation. ( Wind enhances evaporation)

Question 3

TEMPERATURE INCREASE: MORE EVAPORATION BEFORE SATURATION IS REACHED

Answer 3

• At higher temperatures the liquid water molecules will have more kinetic energy (= higher average speed) and as a consequence the rate of evaporation will be higher.
• In this case the dynamic equilibrium between evaporation and condensation will be reached at higher concentrations of water vapour above the water surface.
• For this reason cold air is dryer (contains less water vapour) and warm air is more humid (contains more water vapour)

Humidity - amount of water vapour in the air.