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)
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)
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
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.
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.
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 0C or 31.7 mb at 25 C.
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.
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 0C or 31.7 mb at 25 C.
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:
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.
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).
Relative Humidity We can now define relative humidity as: in words, RH = 100% means air is saturated
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.
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.
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.