The first thing we need to realize is that warm water will
evaporate more rapidly than cool water. You probably
know that already. If a cup of iced tea were set next to
a cup of hot tea you probably be able to tell which was which
by just looking at them. You wouldn't need to touch or
taste the tea or look for ice cubes in the iced tea.
You might notice that one of
the cups of tea was steaming (the cup on the right
above). This would be the hot tea. You're not
actually seeing water vapor. Rather water vapor is
evaporating so quickly that it is saturating the air
above. The air isn't able to accommodate that much
water vapor and some of it condenses and forms a cloud of
steam. That's what you are seeing.
Now we'll redraw the picture and cover both cups so that
water vapor can begin to buildup in the air above the water
in both cups.
Arrows represent the different rates of evaporation.
One arrow is shown evaporating from the cup of cold
water. We'll just assume the warmer water at right is
evaporating 3 times more rapidly. We've arbitrarily
assigned rates of evaporation of 10 and 30 to the water in the
two cups. The actual numbers aren't important, the
important point is that the warm water evaporates more quickly
than the cold water.
Water vapor will start to buildup in the air above each
cup. And, even though it has just evaporated, some of
the water vapor will condense and rejoin the water at the
bottom of each cup. Let's just assume that 1% of the
water vapor molecules will condense (again just a made up
The amount of water vapor in each glass will increase until it
reaches a point where
water evaporation rate =
water vapor condensation rate
for the cup of cold water
10 = 0.01 x water
The 0.01 is 1% expressed in decimal form. Solving
you a water vapor concentration of 1000. The air is
saturated when you reach this point and the RH = 100%.
The saturation water vapor concentration in the air in the
warm cup would be 3000. And again the relative humidity
would be 100%.
The air is saturated in both glasses but there is more water
vapor in the warm glass.
The fact that the rates of evaporation and condensation are
equal when air is saturated (RH = 100%) is something we'll be
using later when we study the formation of
precipitation. Here's a picture of how that would look
inside a cloud.
The air inside the cloud is saturated. The rate of
evaporation from the cloud droplet (2 green arrows) is
balanced by an equal rate of condensation (2 orange
arrows). The RH = 100%. The cloud droplet won't
grow any bigger or get any smaller.
Here's something to test your understanding of this
material. As a matter of fact I'll make it an Optional
Assignment. Turn in your answers to the questions below
at the start of class on Wed., Mar. 20 and you'll receive some
What information can you add to this picture? Is the
water in Glass A WARMER or COLDER than
in Glass B? Is there MORE or LESS
water vapor in the air in Glass A than in Glass B? Is
the relative humidity in each glass MORE than 100%, LESS than
100% or is it EQUAL to 100%. The rates of evaporation
and condensation aren't equal in either glass, so the pictures
will change with time. What will the glasses look once
they have reached equilibrium?
We'll end this section with a table that shows the
dependence of saturation mixing ratio on air temperature.
Note that the value of the
saturation mixing ratio doubles for every 20 F increase in
temperature. The same data are shown in graphical