1. Humidity First let us consider five humidity variables
1. Vapor pressure (e)
2. Mixing ratio (w)
3. Relative humidity (RH)
4. Dewpoint temperature (Td)
5. Wetbulb temperature (Tw)

2. Humidity
Vapor pressure: The partial pressure of water vapor, i.e., that portion of total atmospheric pressure that is due to the presence of H2Ov.
Recall: N2, O2, A, CO2 all have partial pressures
Example: e = 30 mb
Range: 0 to 50 mb (80 mb is extreme)

3. Humidity w = mass H2Ov / mass dry air (in g kg-1)
Mixing Ratio: The ratio of the mass of water vapor to the mass of dry air in a sample.
w = mass H2Ov / mass dry air (in g kg-1)
Example: 10 g kg-1
Range: 0 to 30 g kg-1

4. Saturation Vacuum Time H2O H2O T = 25 C Pressure = 0 mb T = 25 C
We start with liquid water in an evacuated chamber. At some
point we allow for the molecules to move. At first, since there
are none above the water, some of the more energetic water
molecules will leave the liquid and enter the vapor.

5. Saturation Time H2O H2O T = 25 C Pressure = 16 mb T = 25 C
Evaporation continues until the number of molecules that leave
the liquid are the same as the number that return to the liquid.
This is called equilibrium.
At this point, the maximum number of vapor molecules exists
above the liquid at a given temperature. We say that the
vapor is saturated.

6. Saturation H2O T = 25 C Pressure = 31.7 mb
In saturation, the same number of water molecules leave the
liquid as enter it ---- EQUILIBRIUM. The pressure exerted by
the water is called the saturation vapor pressure.
The pressure inside the container is the pressure of the water
vapor molecules alone. This saturation (equilibrium) vapor
pressure is 6.11 mb at 0C or 31.7 mb at 25 C.

7. Saturation Dry Air Time H2O H2O T = 25 C T = 25 C
We can redo this experiment with dry air above the liquid water
in the same container.
We start with no water vapor above the liquid. Water molecules
begin to leave the liquid water and enter the air.

8. Saturation Time H2O H2O T = 25 C T = 25 C
Eventually we come to equilibrium. The saturation vapor
pressure is identical to the previous scenario!
The pressure inside the container is the pressure of the water
vapor molecules plus the pressure of the dry air molecules.
This saturation (equilibrium) vapor of water remains the same
at 6.11 mb at 0C or 31.7 mb at 25 C.

9. Saturation
Vapor Pressure (e) - The partial pressure of the water vapor at a given temperature.
Saturation Vapor Pressure (es) - The partial pressure of the water vapor at saturation at a given temperature
RH = (e / es)  100
RH = (w / ws)  100
Also:

10. Saturation Vapor Pressure
The saturation vapor pressure varies with temperature:
As the temperature
increases, so does the
mean kinetic energy of the
molecules. More can
escape the water surface
and enter the air. The
pressure exerted by these
molecules is higher.
This occurs regardless of
whether air is there or
not.

11. Saturation
Saturation: If there are the maximum number of water vapor (H2Ov) molecules in the vapor phase at a given temperature, we say the vapor is saturated. This depends only on the temperature of the air.
Therefore, for any given temperature (T),
there is a saturation vapor pressure (es),
and a saturation mixing ratio (ws).

12. Relative Humidity We can now define relative humidity as: in words,
RH = 100% means air is saturated

13. Relative Humidity Sample problem:
If T = 240 C and w = 5 g kg-1, what is RH?
Go to a set of tables (Table B.1 in Appendix B) and look up what the saturation mixing ratio (ws) is at T = 240 C. We find it is ws = 20 g kg-1.

14. Dew Point
Dew point temperature (Td): The temperature to which the air must be cooled (at constant pressure and without changing the moisture) for it to become saturated.
Example: T = 240 C, RH = 25%, w = 5 g kg-1
If cooled to 30 C, then ws = 5 g kg-1.
so the dew point temperature is 30 C.

15. Relative Humidity If T = Td , RH = 100% ; the air is saturated.
If T = 200 C and Td = 30 C, then RH = 25%. Notice that this is NOT 30C/ 200C!
The drier the air, the more it has to be cooled for it to reach the dew point.