Vapor Pressure, Equilibrium Vapor
Pressure, and Relative Humidity
I want to take a few moments to talk about condensation evaporation, vapor pressure, and
equilibrium vapor pressure, because those are some important concepts from the lesson, and
they're frequently misunderstood. At the elementary school level and even high school level,
some of these concepts-- especially as they pertain to relative humidity-- get summed up with the
idea that warm air holds more water vapor than cold air. And that's really not true. That's a bad
science explanation. But you can really understand what's going on if you understand what's
going on with condensation and with evaporation.
So let's look at this little experiment here. It's a closed chamber, and let's pretend that at the
beginning, we have only liquid water present. There's no gaseous water vapor present initially.
So that means our vapor pressure here, as indicated on the little gauge, is zero. That's our initial
state.
But as we go on, we have water beginning to evaporate into water vapor, and we start to get a
collection of water vapor molecules in the air over the water. The evaporation is occurring at a
rate that is set primarily by the temperature of the water. So evaporation and temperature are tied
together. As we get those water vapor molecules over the water, some begin to condense back
into liquid water, and the rate of that condensation depends on the amount of water vapor
molecules that are present.
So as time goes on, and we get more and more water vapor present over the water, and our vapor
pressure increases, our condensation rate continues to increase until it equals the evaporation
rate. And when that happens, the vapor pressure is equal to the equilibrium vapor pressure. The
condensation rate is equal to the evaporation rate. That's the state of equilibrium when all those
things are equal.
And when that happens, the relative humidity is equal to 100%. And I think it's important to
realize that condensation and evaporation are going on around you at all times. You just can't see
them all the time, because they're not visible at the individual molecular level. But they're
occurring all the time.
So let's wrap this up with a few key ideas that I want you to be able to tie together. First, vapor
pressure depends on the number of water vapor molecules present, and that is connected to
condensation rates and dew points. Dew point is the meteorological variable that we measure
that is tied to the ideas of vapor pressure and condensation rates.
Equilibrium vapor pressure, on the other hand, that depends on temperature, which is connected
to evaporation rates. So you want to tie those ideas together in your mind.
Relative humidity depends on both the vapor pressure and the equilibrium vapor pressure. So, in
other words, it depends on both dew point and temperature. So that means that relative humidity
is actually not a good indicator of the amount of water vapor present because it depends on
temperature. Relative humidity just indicates how close the rates of condensation and
evaporation are to each other, or, alternatively, the vapor pressure and the equilibrium vapor
pressure, how close the dew point is to the temperature.
Simply by increasing the temperature, you increase evaporation rates and the equilibrium vapor
pressure, and that would act to decrease relative humidity. So relative humidity could go down,
even if you had a fixed amount of water vapor present. If you want to know how much water
vapor is present, the better indicator is dew point temperature, because that depends on the vapor
pressure.