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Extraction of Water From Air

The atmosphere always contains a certain amount of water, usually in its vapor form. There are two processes that can cause this water to be extracted, producing condensation or precipitation. The air can be cooled below its dew point by either adiabatic or diabatic cooling. Both processes will produce similar results but for entirely different reasons.

Atmospheric Pressure
- Air is comprised of atoms, meaning it has mass and weight. At sea level, the weight of the atmosphere is approximately 14.5 pounds per square inch. As altitude is increased, the amount of atmosphere decreases. This results in less atmospheric weight, or pressure. When a parcel of air rises, this reduction in atmospheric pressure causes the air parcel to expand.
Adiabatic Cooling
- Adiabatic cooling involves a change in the temperature of an air parcel without the gain or loss of heat. If the total amount of energy in a parcel of air is the same, that energy can be used for either expansion or to maintain temperature -- but not both. This means that as a parcel of air rises and expands, due to a decrease in atmospheric pressure, its temperature must drop.
Relative Humidity and Dew Point
- Air parcels contain a certain amount of water vapor at any given time. Relative humidity is the ratio of actual water vapor to the amount of water vapor necessary for the air parcel to become saturated. Water vapor is constantly condensing and evaporating within the parcel of air. The dew point represents the temperature at which the air parcel will become saturated, meaning more condensation than evaporation will occur. When the air reaches the dew point, the water vapor will begin to coalesce into larger droplets of liquid water.
Cloud Formation and Precipitation
- Putting these facts together, clouds form when warm moist air rises. This vertical movement causes the air parcels to expand and adiabatic cooling to occur. As the temperature of the air decreases, its relative humidity always increases, meaning it gets closer to its saturation point. When it reaches the dew point, condensation exceeds evaporation and small water droplets begin to combine, or coalesce, into larger drops. The result is condensation or precipitation.
Diabatic Cooling
- Diabatic cooling occurs when the temperature of an air parcel is decreased due to a loss of heat. This can occur when warm moist air flows over a cold body of water, such as a lake. Or, this could occur when air settles over ground that cools overnight. If the air cools to its dew point, it will become saturated and fog will form. This process also occurs when warm moist air comes into contact with a cold surface, such as a mirror or glass. Or, when steam in a pot comes into contact with the cooler lid. The result is the same; condensation occurs and water is extracted from the air.

Nature