1. Lecture 4.2 - Saturation The level you’ve reached by week 4 of the semester

2. Today We need saturation because this is the point at which condensation starts and we can start to form clouds and then precipitation. We can look at this through our outstanding humidity measures

3. We can now define relative humidity as: in words, RH = 100% means air is saturated This is what we are sensitive to Relative Humidity

4. Relative Humidity Trends Relative humidity (RH) indicates air parcel proximity to saturation. Saturation can be achieved, or RH increased, by adding more water or dropping the air temperature. Saturation can be achieved, or RH increased, by adding more water or dropping the air temperature. Dew point is the temperature at which saturation occurs. Dew point is the temperature at which saturation occurs. Figure 5.11

5. Dew Point Dew point temperature (T d ): The temperature to which the air must be cooled (at constant pressure and without changing the moisture) for it to become saturated. Example: T = 24 0 C, RH = 25%, w = 5 g kg -1 If cooled to 3 0 C, then w s = 5 g kg -1. so the dew point temperature is 3 0 C. We get these figures from the tables in the book (appendix D)

6. Relative Humidity and dewpoint If T = T d, RH = 100% ; the air is saturated. If T = 20 0 C and T d = 3 0 C, then RH = 25%. Notice that this is NOT 3 0 C/ 20 0 C! The drier the air, the more it has to be cooled for it to reach the dew point.

7. Measuring Humidity Instruments: –Hygrometers -- The physical characteristics of a substance changes when it gets moist. A common example is that human hair elongates and contracts with variations in moisture. human hair paper

8. Measuring Humidity Wet bulb temperature (T w ): the temperature to which the air must be cooled (at constant pressure) by evaporating water into the air for the air to become saturated. An instrument which measures T w is called a psychrometer. –A thermometer with a wet, muslin wick covering the bulb –Wick/bulb cools by evaporation

9. Measuring Humidity If T = T w = T d, RH = 100%. Usually T > T w > T d, since moisture is being added.

10. Saturation There are two fundamental ways of saturating a parcel of air –The first is to add more moisture to the parcel at constant temperature and constant pressure. –The second is to cool the parcel until the saturation vapor pressure equals the vapor pressure of the parcel without adding moisture. Then we may get CONDENSATION

11. Review of 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.

12. Saturation Vacuum H2OH2O T = 25  C Pressure = 0 mb H2OH2O T = 25  C Pressure = 1 mb Time 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.

13. Saturation T = 25  C Pressure = 16 mb H2OH2O T = 25  C Pressure = 31.7 mb Time 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. H2OH2O

14. Saturation 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. H2OH2O T = 25  C Pressure = 31.7 mb

15. Saturation Dry Air H2OH2O T = 25  C H2OH2O Time 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.

16. Saturation H2OH2O T = 25  C Time 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. H2OH2O T = 25  C

17. Saturation Vapor Pressure (e) - The partial pressure of the water vapor at a given temperature. Saturation Vapor Pressure (e s ) - The partial pressure of the water vapor at saturation at a given temperature RH = (e / e s ) x 100 RH = (w / w s ) x 100 Also:

18. Therefore, for any given temperature (T), there is a saturation vapor pressure (e s ), and a saturation mixing ratio (w s ). Saturation Saturation: If there are the maximum number of water vapor (H 2 O v ) molecules in the vapor phase at a given temperature, we say the vapor is saturated. This depends only on the temperature of the air.

19. Relative Humidity and Comfort Unsaturated air may absorb more water from the evaporation of human sweat. The departure of fast moving, and by definition higher temperature, water molecules into the vapor phase cools the human skin. The departure of fast moving, and by definition higher temperature, water molecules into the vapor phase cools the human skin. In winter, this process can make a dry house extra chilly. In winter, this process can make a dry house extra chilly. Swimming, chilled water cooling towers, spray bottles, downdrafts, sweat.

20. Heat Index & Safety Figure 5.16 Human perception of temperature is distinct from measured air temperature, and is particularly different at higher humidities when the human body is less efficient at sweating and self- cooling. On hot days, fans that move saturated air away from the skin help humans avoid unwanted heat syndromes. On hot days, fans that move saturated air away from the skin help humans avoid unwanted heat syndromes.