1. AOSC 200 Lesson 4

2. MOISTURE WATER VAPOR CONSTITUTES ONLY A SMALL FRACTION OF THE ATMOSPHERE. VARIES FROM 0 TO 4 PER CENT HOWEVER WATER IS PROBABLY THE MOST IMPORTANT GAS IN THE ATMOSPHERE FOR UNDERSTANDING ATMOSPHERIC PROCESSES. THE SOURCE OF ATMOSPHERIC WATER IS EVAPORATION

3. p. 83 Satellite picture of clouds over North America, early January, 1998 Yellow and red in this picture are clouds which covered Canada. These clouds produced freezing rain and produced up to 4 inches of ice Extensive damage was done to transmission lines.

4. p. 83 Crumpled steel electrical transmission towers – Canada, Jan. 1998

6. Fig. 4-1, p. 84 Sequence of events leading to saturation of water vapor in air

7. Fig. 4-3, p. 87 Observations of vapor pressure as a function of temperature

8. Fig. 4-5, p. 83

9. HUMIDITY HUMIDITY DESCRIBES THE AMOUNT OF WATER VAPOR IN THE AIR. HUMIDITY IS DESCRIBED QUANTITATIVELY AS, ABSOLUTE HUMIDITY, MIXING RATIO AND RELATIVE HUMIDITY SATURATION IS ACHIEVED WHEN THE NUMBER OF WATER VAPOR MOLECULES LEAVING A WATER SURFACE IS EQUAL TO THE NUMBER RETURNING FROM THE ATMOSPHERE TO THE WATER SURFACE. SATURATION VAPOR PRESSURE IS THE PRESSURE EXERTED BY THE WATER VAPOR AT SATURATION. ABSOLUTE HUMIDITY IS THE MASS OF WATER PER UNIT VOLUME. UNITS ARE USUALLY GRAMS PER CUBIC METER. MIXING RATIO IS THE MASS OF WATER VAPOR IN AN UNIT MASS OF AIR. USUALLY IN GRAMS PER KILOGRAM. RELATIVE HUMIDITY IS THE ACTUAL AMOUNT OF WATER VAPOR IN THE AIR OVER THE AMOUNT OF WATER VAPOR REQUIRED FOR SATURATION.

10. Fig. 4-7, p. 93

11. Curvature Effect On the surface of a water droplet each water molecule is pulled down by fewer neighbor molecules. This curvature effect makes it easier for molecules to evaporate from a droplet. Hence the saturated vapor pressure will be higher. It now takes a lower temperature before the water will condense to form a droplet– super-saturation Cloud chamber. In practice droplets form around condensation nuclei.

12. Fig. 4-2, p. 86 Atmospheric water vapor comes from the earth’s surface, hence its mixing ratio drops with altitude. The mixing ratio also depends on the temperature – hence the change with latitude.

13. Fig. 4-4, p. 90 Assume that the absolute humidity is constant over the 24 hour period. In the early morning the temperature is low, the saturation vapor pressure is small, the relative humidity is large. In the afternoon the temperature is high, the saturation vapor pressure is high, the relative humidity is low.

14. RELATIVE HUMIDITY RELATIVE HUMIDITY CHANGES AS DAILY TEMPERATURE CHANGES. IT WILL CHANGE FROM ONE LOCATION TO ANOTHER IT CHANGES WHEN AIR MOVES VERTICALLY IN THE ATMOSPHERE. DAILY VARIATION OF TEMPERATURE AND RELATIVE HUMIDITY. HOWEVER THE WATER VAPOR CONTENT OF THE AIR CAN STAY THE SAME. DEW POINT IS THE TEMPERATURE AT WHICH WATER VAPOR WILL CONDENSE OUT OF THE ATMOSPHERE AS DEW. ON SOME DAYS THE WATER VAPOR GOES DIRECTLY TO ICE. THIS TEMPERATURE IS CALLED THE FROST POINT.

15. Dew and Frost Dew is formed when the temperature at the surface falls below the dew point. Frost is one of the few examples of deposition in nature.

16. Fig. 4-5, p. 91 When the temperature of the air around this web was cooled to below the dew point temperature, dew formed, making the web more visible

17. Temperature (°C) -10 20 Relative Humidity 25752575 Mixing Ratio (g/kg) 0.451.353.6711.15 Vapor Pressure (mb) 0.722.165.8717.60 Sat. vapor pressure (mb) 2.88 23.47 Dew point Temp. (°C) -26.2-13.5-0.515.6 Dew point depression (°C) 16.23.520.54.4

18. Average surface dew point temperature for January.

19. Average surface dew point temperature for July

20. CONDENSATION AND DEPOSITION CURVATURE EFFECT – EVEN IF AIR IS SATURATED OVER A FLAT SURFACE, IT MAY NOT BE FOR A CURVED SURFACE. SUPERSATURATION – RELATIVE HUMIDITY CAN BE ABOVE 100% WITHOUT CONDENSATION NUCLEATION – DROPLETS USUALLY FORM AROUND PARTICLES – CONDENSATION NUCLEI CONDENSATION NUCLEI CAN BE HYDROSCOPIC, HYDROPHOBIC, ICE NUCLEI.

21. Box 4-1, p. 89 Heat index table One’s body temperature is a balance between the rate at which it receives energy and the rate that it loses energy by evaporating water from the skin. In high relative humidity less water evaporates and the body temperature is higher. The heat index is the effective temperature that the body feels.

23. Fig. 4-11, p. 97 Advection fog

24. Fig. 4-12, p. 98 Steam Fog

25. FOG FORMATION Fog is defined as a cloud with its base at or near the ground Fogs result when air is cooled or by the addition of water vapor to cause saturation Radiation fog – cooling of the earth’s surface by emitting thermal radiation Advection fog – warm and moist air blown over a cool surface. need turbulence at the surface Evaporation/steam fog – air picks up additional water over water surfaces Upslope fog – air is cooled as it flows up a slope

26. Fig. 4-13, p. 99 Four mechanisms that cause air to ascend

27. Lifting Mechanisms that form Clouds Lifting condensation level Orographic lifting Frontal lifting Convection Convergence

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29. Fig. 4-16, p. 102 Major cloud types arranged by altitude.

32. Precipitation Growth in Warm Clouds Cloud droplets are typically 10 microns in size. small raindrops are typically 1000 microns (almost one million droplets) How does one go from one to the other? Simplest explanation is that the droplet grows by condensation of water vapor on its surface. But this would take several days. One way for raindrops to form is by a processes known as collision-coalescence. This process requires lots of droplets - high absolute humidity - seen only in the tropics.

33. Collision-Coalescence Warm cloud because process because it only occurs in the tropics. As the cloud droplet is formed it is pulled down by gravity but then is moved upward by the rising air within the cloud. Hence within a cloud one has droplets moving in all directions and they can collide and form larger droplets by coalescing. Once a drop grows to a size when the force of gravity exceeds the uplift from the rising air, the drop moves downward through the cloud picking up other droplets as it falls.

34. Collision Coalescence

35. Fig. 4-31, p. 114 Collision- coalescence process

36. Precipitation Growth in Cold Clouds Outside of the tropics there are just not enough cloud droplets to form rain drops by the collision- coalescence process. A Swedish atmospheric physicist, Dr. Bergeron, would vacation in the fall in the mountains. He often took an early morning walk along a path that led through a pine forest and encountered uplift fogs. He noted that on the days when the temperature was above 0 C, the fog went all the way to the ground, whereas if the temperature fell below -10 C, the fog lay above the tops of the trees.

37. Bergeron walk

38. Why? He further noted when the temperature fell below -10 C, that the pine needles were covered with ice. The reason is that the vapor pressure of water above ice is less than the vapor above water.

39. Fig. 4-33, p. 116 Attraction of water vapor to ice versus water

40. Fig. 4-35, p. 117 Saturation vapor pressure over ice and over water

41. Fig. 4-34, p. 116

42. Accretion and Aggregation It should be noted that the ice crystal growth gives a ‘snow flake’ which eventually will also begin to fall and then rise in the updraft. These ice crystals then begin to aggregate into larger snowflakes. The ice crystal can also collide with a supercooled water droplet which instantly freezes - accretion. As the ice crystals descend at some point the temperature can rise above 0 C, and the crystal melts to form a raindrop.

43. Fig. 4-32, p. 115 Process of aggregation

44. Aggregation

45. Fig. 4.27

46. Fig. 4.33 These picture shows fall-streaks. They consist of ice particles that have fallen out of a cloud and evaporate before they reach the ground.

47. Fig. 4.38 Steps in the formation of the precipitation types

48. Fig. 9.13 Warm Front

49. Fig. 4-37, p. 118

50. FORMS OF PRECIPITATION RAIN - DROPLETS OF WATER GREATER THAN 0.5 MM IN DIAMETER. WHEN DROPLETS SMALLER THAN 0.5 MM CALLED DRIZZLE. MUCH RAIN STARTS OUT ALOFT AS ICE CRYSTALS SNOW - ICE CRYSTALS. IF AIR IS COLD (LOW HUMIDITY), WE GET LIGHT AND FLUFFY SNOW (POWDER). IF AIR IS WARM THAN ABOUT -5 CELSIUS, THEN WE GET WET SNOW (GOOD FOR SNOWBALLS). SLEET - SMALL PARTICLES OF ICE. RAINDROPS ENCOUNTER FREEZING AIR ON DESCENT. IF FREEZING NOT COMPLETE - FREEZING RAIN. HAIL - LAYERS OF ICE FORM AS THE HAILSTORM TRAVELS YUP NAND DOWN IN A STRONG CONVECTIVE CLOUD RIME - FORMED BY FREEZING OF SUPERCOOLED FOG ON OBJECTS.

51. Fig. 4-42, p. 122 The effects of airflow over a mountain

52. Clouds and Precipitation near Mountains As air ascends mountain it cools adiabatically, clouds form, and precipitation occurs. Above this altitude the relative humidity stays at 100% At the peak of the mountain the absolute humidity is determined by the saturation vapor pressure at - 12C. As the air descends its absolute humidity remains the same as at the peak

53. Clouds and Precipitation near Mountains As the air descends it is compressed, so it warms Hence the saturation vapor pressure will increase, and the relative humidity will decrease The net effect of the air ascending and descending the mountain is that the air becomes drier and warmer. On the island of Hawaii, the west side of the coast (westerly winds) has rain forests, the eastern side has deserts.

54. Table 4-4, p. 123