Atmospheric Moisture & Water Circulation

Atmospheric Moisture:
Water may exist in any of several phases: solid, liquid or gas, depending upon temperature and pressure. The terms evaporation and condensation are used to describe water in transition between the gaseous and liquid states. The terms freezing and melting are used to describe water in transition between the liquid and solid states.

Sublimation is a change of phase directly from the solid state to the gaseous state.
Deposition is a change of phase directly from the gaseous state to the solid state.

A saturation state is a state of equilibrium between water molecules in transition between the liquid phase and gaseous phases. The temperatures and pressures at which saturation states occur are of great importance in determining the course of most weather-related phenomena.

As we will see, water vapor, while a very small constituent of the atmosphere (percentage wise), plays a huge role in determining the course of weather and climate.

Winds enhance evaporation by transporting water vapor molecules in the air away from the site where the water is entering the air, preventing the air from becoming saturated at that site. Higher temperatures also enhance evaporation.

Water molecules in the liquid and gaseous states exist in a state of rapid motion, zipping back and forth as a result of collisions with other water molecules. In a parcel of water at a given temperature there will be a distribution of energies associated with various water molecules (i.e., not all of the water molecules will possess the same energy). The higher the temperature, the higher the mean value of the energy distribution, or the higher the average energy of each water molecule. The greater the energy of an individual water molecule, the more vigorous its motion. Water molecules at higher energies move around with greater average speeds (know as rms speeds).

Whenever water exists in the liquid state there will be at least a few molecules that have high enough energies to make the transition from the liquid to the vapor state. When the air is warm, however, many water vapor molecules will possess high enough energies to bounce around for only a short period of time before escaping the liquid state.

When the air is cooled, however, lower rms speeds allow water molecules to stick to certain objects (known as condensation nuclei) after a collision. When water sticks to condensation nuclei in the air liquid cloud droplets begin to form. Condensation is more likely when the air is cooled both because water vapor molecules can "stick" to condensation nuclei, and because lower rms speeds allow more water vapor molecules to return to the liquid state.

Circulation of Water in the Atmosphere:
The transfer of water between land, ocean and atmosphere is accomplished by a number of natural processes. Evaporation places water into the atmosphere. Transpiration, the transfer of water into the atmosphere by biological systems (primarily plants) also places water into the atmosphere. Evaporation and transpiration work together in continental areas and account for roughly 15% of the total amount of water placed into the atmosphere. The other 85% come directly from the oceans via evaporation.