1. NATS 101 Lecture 6 Part A Atmospheric Moisture Turn On Clickers

2. Hydrological Cycle 85% of water vapor in atmosphere evaporates from oceans. Ahrens Fig. 4.1 85% 15% 50% Ultimate source of all fresh water About 50% of precipitation that falls over land is runoff, and the other 50% is transpired/evaporated. Water vapor molecules reside in atmosphere for one week. 1 week

3. Colorado River Reservoir Levels

5. Lake Mead http://www.arachnoid.com/NaturalResources/image.php?mead

6. Lake Powell http://www.usbr.gov/uc/water/

7. Humid Air Humid air is a mixture of molecules that make up dry air (mostly N 2 and O 2 ) and lighter water vapor (H 2 O) molecules. Each type of molecule contributes a fraction of total air pressure, or a partial pressure, proportional to the number molecules per unit volume. The partial pressure of water vapor is termed the vapor pressure.

8. Saturation Vapor Pressure The partial vapor pressure at which the rate of evaporation equals the rate of condensation in a closed system is called the saturation vapor pressure or SVP. The SVP effectively denotes the maximum water vapor that air the can “hold”. SVP depends strongly on temperature. Vapor pressure and SVP provide a measure of the actual water vapor content and the air’s potential capacity, respectively.

9. SVP depends on temperature. As water warms, more molecules have fast enough speeds to escape into the air. Williams p62 dry E>C saturated E=C warmed E>C cooled E<C Concept applies to an ice surface too. SVP over ice is lower because water molecules are bonded more tightly to ice. For temperatures of interest, some water molecules are energetic enough to escape into atmosphere. So SVP>0.

10. SVP and Temperature SVP almost doubles with a 10 o C warming SVP and T Graph Supercooled water droplets can exist to temps of -40 o C For temps below 0 o C, SVP runs 10%-30% lower over ice Ahrens Fig. 4.5 12 23 42

11. Relative Humidity Air with a RH=100% is said to be saturated. RH depends on air temperature (SVP). RH changes by either changing air’s water vapor content or the air’s temperature. SVP and T Graph

12. Relative Humidity The RH for constant water vapor content can fluctuate greatly during the course of the day solely from the temperature changes Ahrens Fig. 4.6

13. Relative Humidity (Ahrens, Appendix B) Consider air that starts saturated at 0 o C Temp VP SVP RH 0 o C6 mb 6 mb100% 10 o C6 mb12 mb 50% 20 o C6 mb22 mb 26% 30 o C6 mb40 mb 14% 40 o C6 mb70 mb 8%

14. Dew Point (DP) DP-temperature to which air must be cooled at constant pressure to become saturated. Higher DP => Higher water vapor content. DP is a excellent indicator of the actual water vapor content since air pressures vary very little along the earth’s surface. DP is plotted on surface weather maps. DP depression (Temp-DP) is plotted aloft.

15. Arizona Dew Point Ranges Vapor Pressure Dew Point 24 mb 20 o C 12 mb 10 o C 6 mb 0 o C 3 mb-10 o C

16. dry thermometer Wet Bulb Temp -Lowest temp to which air can be cooled by evaporation of water into it. Warmer than dew point since moisture is being added to air which raises dew point. Measured with sling psychrometer. Wet Bulb Temperature wet bulb Ahrens, Fig 4.9

17. Wet Bulb Temperature (Ahrens, Appendix D) Wet bulb temperature is about 30-40% of the way from the dew point to the temperature for surface conditions that characterize AZ Application-Wet bulb temp gives maximum possible efficiency for a swamp cooler MonthMAX Dew PWet Bulb June100 o F 37 o F 65 o F July100 o F 63 o F 75 o F

18. Heat Index Humidity reduces the rate at which sweat evaporates. Thus, the cooling rate is lowered. Tucson Record MAX Rocky Pt Summer July MAX June MAX Ahrens, Fig 4.8

19. Humid Air is Less Dense Williams, p72

20. 0.5% lighter Williams, p72

21. Summary: Moisture Water vapor comes from the evaporation of sea water and resides in atmo. for ~1 week. Air has a saturation level for water vapor Saturation level depends on air temperature Humid air is less dense than dry air Water vapor content can be quantified by RH, dew point temp, wet bulb temp

22. Summary: Moisture Air has a saturation level for water vapor, beyond no additional vapor can be added Saturation level depends on air temperature Actual water vapor content of air can be quantified by several measures Vapor Pressure, Saturation Vapor Pressure, Relative Humidity, Dew Point, Wet-Bulb

23. Summary: Moisture Measures Saturation Vapor Pressure (SVP) - highest possible partial pressure for water vapor Vapor Pressure - actual partial pressure of water vapor Relative Humidity – is the ratio actual content saturation value

24. Summary: Moisture Measures Dew Point (DP) - temperature to which air must be cooled at constant pressure to become saturated. Plotted on WX maps. Wet Bulb Temperature - lowest temperature to which air can be cooled by evaporation of water. Higher than DP since water vapor is added to air. Temp ≥ Wet Bulb Temp ≥ Dew Point

25. Next Class Assignment Fog and Clouds Ahrens - Reading 3 rd Pg: 87-105, 109-118 4 th Pgs: 89-108, 111-121 5 th Pgs: 91-108, 111-120 Homework04 - D2L (Due Monday Feb. 22) 3 rd -Pg 105: 4.13, 14, 15, 16 4 th -Pg 108: 4.13, 14, 15, 16 5 th -Pg 108: 4.13, 14, 15, 16 Do Not Hand In 4.16

26. NATS 101 Lecture 6 Part B Condensation Cloud tracks caused by cloud condensation nuclei from ship exhaust http://mm04.nasaimages.org

27. Cloud Condensation Nuclei Small, airborne particles are necessary on which water vapor can condense to produce cloud droplets Without such particles, RH>100% would be needed to produce clouds Such surfaces are called Cloud Condensation Nuclei (CCN) CCN are light and stay suspended for days

28. Cloud Condensation Nuclei Sources Dust, volcanic ash, smoke, soot, salt, sulfate particles Concentrations 1,000-10,000 per cc Highest over cities Highest at surface Ahrens, Meteorology Today, 5th Ed.

29. Cloud Condensation Nuclei Water-Seeking Salt Sulfuric acid Nitric acid Water-Repelling Oils Gasoline Waxes Condensation can occur on hygroscopic CCN for RH<100% Accounts for Haze when the RH approaches 60-80% Ahrens, Meteorology Today, 5th Ed.

30. Haze over Melting Snow Ahrens, Meteorology Today, 5th Ed.

31. Condensation When air becomes supersaturated from either the temperature cooling or the addition of water vapor, water condenses onto CCN as small cloud droplets Diameters of droplets are 2-20 microns, (10-100 times smaller than human hair) Concentrations are 50-1,000 droplets per cc

32. Size of Cloud Droplets Williams, The Weather Book, p73 100 times 0.2

33. Summary: Condensation Condensation Can occur by cooling or moistening of air CCN permit condensation at RH’s < 100% Small (<0.2 to 1 microns) airborne particles Responsible for Haze formation at RH < 100%

34. Next Class Assignment Fog and Clouds Ahrens - Reading 3 rd Pg: 87-105, 109-118 4 th Pgs: 89-108, 111-121 5 th Pgs: 91-108, 111-120 Homework04 - D2L (Due Monday Feb. 22) 3 rd -Pg 105: 4.13, 14, 15, 16 4 th -Pg 108: 4.13, 14, 15, 16 5 th -Pg 108: 4.13, 14, 15, 16 Do Not Hand In 4.16