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Water Vapor and Temperature
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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Update: June 2012
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