Download Saturation: A Brief Review

Saturation: A Brief Review
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For any body of water, molecules are continually leaving and
returning
This implies three possible states:
leaving > returning
net evaporation
returning > leaving
net condensation
leaving = returning
saturation
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The two properties controlling the system are the temperature of
the water and the vapor content of the air
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Temperature controls the rate at which molecules leave the
body of water
– Higher temperature means more molecules escaping
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Vapor content controls the rate at which molecules return to
the body of water (at least mainly)
– Higher vapor content means more molecules returning
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So whether we have net evaporation, net condensation, or
saturation depends on the interplay between temperature and
vapor content
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The two properties controlling the system are the temperature of
the water and the vapor content of the air
●
Temperature controls the rate at which molecules leave the
body of water
– Higher temperature means more molecules escaping
●
Vapor content controls the rate at which molecules return to
the body of water (at least mainly)
– Higher vapor content means more molecules returning
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So whether we have net evaporation, net condensation, or
saturation depends on the interplay between temperature and
vapor content
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The relative humidity (RH) measures how close we are to
saturation. Specifically:
In terms of relative humidity, the three possible states look like
But note that while RH can be much less than 100%, it never really
gets much greater than 100%. (Why?)
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Finally, remember that cloud droplets can exist only if the air is
saturated or super-saturated---i.e., if RH = 100% or greater
– Otherwise the droplets would just evaporate!
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Suppose we have air that happens to be sub-saturated (i.e.,
RH < 100%). Then to form a cloud, we'll first need to get RH
up to 100%..........but how?
RH =
actual vapor content
content needed for saturation
x 100 %
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Well, consider two systems: one warmer and one colder. In the
warm system, water molecules escape from the water layer at a rapid
rate, while in the cold system, they escape more slowly.
To balance things out, the warm system's gonna need a lot more
water molecules in the air, so that the rate of return matches the rate
of escape. On the other hand, the cold system can balance with
fewer molecules.
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Ergo, the amount of vapor
needed for saturation is a
function of temperature!
Specifically, warmer air
needs more vapor to be
at saturation.
We often summarize this by
saying warmer air holds
more water vapor (at
saturation).
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Ok, now back to the relative humidity:
RH =
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actual vapor content
content needed for saturation
x 100 %
If we increase the temperature, then the amount of vapor needed
for saturation goes up, which means the RH......
On the other hand, if we decrease the temperature, the amount
of vapor needed for saturation goes down, so RH......
RH =
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actual vapor content
content needed for saturation
x 100 %
Note that if we decrease the temperature far enough, eventually
we'll hit saturation (RH = 100%)
Rising Motion and Clouds
What's all this have to do with clouds?
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Well.....it turns out that as air rises in the atmosphere, it's
temperature changes---specifically, it becomes colder
– Basically, rising air expands, which costs the air some
energy......but we'll talk about that later when we get to
stability
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In any case, as the air rises, it cools; and as it cools, it's RH
increases
If the air rises high enough and becomes cold enough, then
eventually the air becomes saturated (RH = 100%), allowing
clouds to form
There's various ways to get rising air and clouds.......
surface heating / convection
ascent over mountains
convergence into low pressure
lifting along fronts
mountains produce upward motion, and hence clouds
rainfall in WA state