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Name: Vicki
Status: educator
Age: N/A
Location: N/A
Country: N/A
Date: 2000-2001


Question:
One of my students (college level introductory physical geography) asked why atmospheric capacity to contain water vapor increase with temperature. I could whip out the formula, but that wouldn't answer her question.

Why does water vapor capacity (versus water vapor content) increase with air temperature, and does this hold true for other gases and the atmosphere?


Replies:
Actually, it doesn't have anything to do with the air at all. It's just a matter of the equilibrium between the water (liquid phase) and its vapor (gas phase). As you and your student know, a liquid and vapor phase of the same substance in equilibrium are actually in dynamic equilibrium, in which molecules in the gas phase are combining and condensing to form liquid, and molecules in the liquid phase are evaporating to form gas. The rates of these processes are the same, so the total proportions in the two phases remain constant.

When the temperature increases, that means that the molecules in both phases have more kinetic energy. This makes it more difficult for gas-phase molecules to condense to form the liquid (they bounce apart instead of sticking together), and easier for liquid-phase molecules to evaporate (they can fly out of the liquid phase more readily). So, more liquid-phase molecules move into the gas phase than the reverse. The concentration of the substance in the liquid phase increases until the molecules being so close together that the condensation process again is as fast as the evaporation process. This is the new equilibrium concentration of the substance in the vapor phase, which is usually expressed as a vapor pressure. Because of this interplay between condensation and evaporation, the vapor pressure of any substance is greater at higher temperatures.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


Vicki,

The capacity of air to hold water vapor is a function only of the temperature of the air (a reflection of the kinetic energy of the molecules and atoms in the air).

The higher the temperature, the greater the capacity to hold water vapor (or, in other words, the more water vapor can be held in the same volume without condensing).

Water content is best described as density of water vapor, the mass of water per volume, which can vary widely depending on atmospheric pressure and temperature.

In the atmosphere, a parcel of air can change volume, and thus temperature and water vapor capacity; for the same mass of air, the volume will be greater in lower atmospheric pressures and smaller in higher atmospheric pressures. As air parcels rise, they cool, thereby decreasing their water vapor capacity (that is why clouds normally form well above the Earth's surface). As air parcels drop, they warm, thereby increasing their water vapor capacity.

What is important, is that the water vapor goes through a change in state (to liquid, via condensation) when the kinetic energy drops to a level where the water molecules can not move fast enough to remain separate from each other. The molecules no longer have enough energy to bounce off of each other and so they coalesce into water droplets.

A good analogy would be if you compared what it would be like for two people to be in a small room (lots of space to move around and you could run fast enough to bounce off of each other if you collided) as opposed to there being 250 people in the room (packed so tight that you could not get up enough speed to bounce off of each other).

Yes, this is true for all gases. However, the activity level (again, temperature reflecting the kinetic energy of the gas) at which individual gases become liquid varies greatly. This scales roughly by molecular weight. Most of the gases in the atmosphere have higher molecular weights (nitrogen 28, oxygen 32) than water (18) and thus require much lower temperatures to change state to the liquid form.

An interesting side-light to the process is that when water vapor changes state to liquid, the water vapor molecules give up the kinetic energy that they had when they were moving. This warms the remaining free molecules in the air and tends to delay further condensation. However, this process actually enhances condensation of water vapor in the atmosphere because a warmed parcel of air rises more rapidly, thus expands more rapidly, thus cools more rapidly, and thus condenses more rapidly than if the parcel had stayed in one place and the temperature were decreased. The enormous growth of towering cumulonimbus clouds demonstrates this dramatically.

David Cook
Meteorologist working in ER Division
Argonne Nat. Lab.


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