Ask A Scientist© Weather Archive

 
>
> >  name       Vicki
> >  status     educator
>
> >  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?
>
>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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