1. + Moisture and Stability Chapter 4

2. + The Hydrologic Cycle Hydrologic Cycle: the circulation of Earth’s water supply The cycle illustrates the continuous movement of water from: The oceans to the atmosphere The atmosphere to the land The land back to the sea

3. + The Water Cycle

4. + The Importance of Water Water Vapor: gas that can change from one state of matter (solid, liquid or gas) to another at the temperatures and pressures experienced on Earth The processes that change that state of matter of water include: Evaporation (liquid to gas) Condensation (gas to liquid) Melting (solid to liquid) Freezing (liquid to solid) Sublimation (solid to gas) Deposition (gas to solid)

5. + States of Matter Latent Heat: as water changes from one state to another latent heat is either released or absorbed Calories: measurement of heat energy involved in the change of state of water Melting: Heat absorbed (80 calories) Freezing: Heat released (80 calories) Evaporation: Heat absorbed (600 calories) Condensation: Heat released (600 calories) Sublimation: Heat absorbed (680 calories) Deposition: Heat released (680 calories)

6. + Humidity The general term used to describe the amount of water vapor in the air Methods to express humidity Absolute humidity: mass of water vapor in a given volume of air Vapor pressure: pressure attributable to water vapor content Relative humidity: ratio of the air’s actual water vapor content compared with the amount of water vapor required for saturation at a specific temperature Dew Point: temperature needed in order for air to reach saturation.

7. + Relative Humidity RH: relates the amount of water vapor in the air to the maximum possible at the current temperature More water vapor exists in warm air than in cold air RH = (specific humidity/saturation specific humidity) x 100% RH depends on both moisture content and the air temperature Adding moisture to the air while keeping the temperature constant increases the relative humidity Removing moisture lowers the relative humidity

8. + Water Vapor Content Changes When water vapor content of air remains at a constant level, a decrease in air temperature results in an increase in relative humidity and an increase in temperature causes a decrease in relative humidity In nature, there are three major ways that air temperature changes to cause changes in RH Daily changes in temperature Temperature changes that result as air moves horizontally from one location to another Changes caused as air moves vertically in the atmosphere.

9. + Relative Humidity changes throughout the day

10. + Relative Humidity In (a), the temperature is 14 o C and has a saturation specific humidity of 10g of water vapor per kilogram of air. If the actual specific humidity is 6g the RH is 60% In (b) the specific humidity is still 6g, but the higher temperature of 25C results in a greater saturation specific humidity. The RH is less in (a) even though the amount of water vapor is the same

11. + Relative Humidity

12. +

13. + Relative Humidity Practice If a parcel of air at 25 o C contains 10 grams of water vapor per kilogram of air, what is its relative humidity? Step One: determine how much water 25 o C can hold. (check chart on page 105) 20 grams Step Two: take the amount of water vapor in the air and divide it by the amount it can hold 10g/20g = 50% relative humidity Temp o C ( o F)Saturation Mixing Ratio -400.1 -300.3 -200.75 -102 03.5 55 107 1510 2014 2520 3026.5 35 4047

14. + Relative Humidity Practice If a parcel of air at 35 o C contains 5 grams of water vapor per kilogram of air, what is its relative humidity? Step One: determine how much water 35 o C can hold. 35 grams Step Two: take the amount of water vapor in the air and divide it by the amount it can hold 5g/35g = 14% relative humidity Temp o C ( o F)Saturation Mixing Ratio -400.1 -300.3 -200.75 -102 03.5 55 107 1510 2014 2520 3026.5 35 4047

15. Temp o C ( o F)Saturation Mixing Ratio -400.1 -300.3 -200.75 -102 03.5 55 107 1510 2014 2520 3026.5 35 4047

16. + Relative Humidity Practice If a parcel of air at 15 o C contains 5 grams of water vapor per kilogram of air, what is its relative humidity? Step One: determine how much water 15 o C can hold. Step Two: take the amount of water vapor in the air and divide it by the amount it can hold Temp o C ( o F)Saturation Mixing Ratio -400.1 -300.3 -200.75 -102 03.5 55 107 1510 2014 2520 3026.5 35 4047

17. + Relative Humidity Practice If a parcel of air at 10 o C contains 5 grams of water vapor per kilogram of air, what is its relative humidity? Step One: determine how much water 10 o C can hold. Step Two: take the amount of water vapor in the air and divide it by the amount it can hold Temp o C ( o F)Saturation Mixing Ratio -400.1 -300.3 -200.75 -102 03.5 55 107 1510 2014 2520 3026.5 35 4047

18. + Water Saturation When air is saturated the pressure exerted by the water vapor increased. Saturation produces a balance between the number of water molecules leaving the surface of the water and the number of returning Because the saturation vapor pressure is temperature dependent, at high temperatures more water vapor is required for saturation to occur.

19. + Saturation vs. Temperature.

20. + Dew Point The temperature to which the air must be cooled to become saturated An expression of water vapor content The closer the air temperature is to dew point indicates lots of water in the air…humid day

21. Where in the US is it most humid? Where is it most dry?

25. + Measuring Humidity Sling Psychrometer: instrument for measuring humidity Wet Bulb Depression: depends on the moisture of the air Dry Bulb Depression: ambient air temperature

28. The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the dew point temperature.

29. The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the relative humidity.

30. + Measuring Humidity Amount of coolness that takes place is directly proportional to the dryness in the air Dryer the air, the greater the cooling Larger the difference between the wet and dry bulb the lower the humidity; the smaller the difference the higher the humidity If the air is saturated, no evaporation will occur and the 2 thermometers will have identical readings

31. + Adiabatic Temperature Change As air is heated it expands becoming less dense, and as a result, lighter. Because it is lighter, it rises upwards above the cooler air. As it does so, this air continues to expand. This is because there is less pressure higher in the atmosphere, allowing the air molecules to spread out more. Temperature changes but no heat is added to or removed from a substance are said to be adiabatic Condensation occurs when water vapor is cooled enough to change to a liquid – produces dew, fog, or clouds

32. + Adiabatic Temperature Change Dry Adiabatic Lapse Rate (DALR): The rate at which a rising parcel of of unsaturated air cools 1.0 o C/100m (5.5 o F/1000ft) Saturated Adiabatic Lapse Rate (SALR): the rate at which saturated air cools 0.5 o C/100m (3.3 o F/1000ft) Lifting Condensation Level (LCL): the altitude at which a parcel of air cools sufficiently and condensation/deposition commences.

33. + Dry Adiabatic Lapse Rate

35. + Processes that Lift Air Air resists vertical movement. Air near the surface stays near the surface, air up high tend to remain up high Four mechanisms that cause air to rise Orographic Lifting Frontal Wedging Convergence Localized convection lifting

36. + Orographic Lifting Occurs when elevated terrains such as mountains act as barriers to the flow of air As air ascends, adiabatic cooling often generates clouds and precipitation Rain Shadow Desert: as air reaches the leeward side of the mountain, much of the moisture has been lost. If the air descends, it warms adiabatically making condensation and precipitation unlikely

37. + Orographic Lifting

38. +

39. +

40. + Rainshadow Effect

41. + Rainshadow Effect California’s Elevation, Great Basin

42. + Frontal Wedging Masses of warm and cold air collide to produce fronts The cooler, denser air acts as a barrier over which the warmer less dense air rises (this is frontal wedging) Mid-latitude cyclones

43. + Frontal Wedging

44. + Convergence Whenever air in the lower troposphere flows together, lifting happens. When air flows in from more than one direction, it has to go somewhere

45. + Convergence

46. + Localized Convection Lifting Unequal heating of Earth’s surface may cause pockets of air to be warmed more than surrounding air. Thermals: pockets of rising hot air Warm parcels of air rise above the lifting condensation level, clouds form, which can bring about mid-afternoon showers.

47. + Convection lifting

48. + Atmospheric Stability Chapter 4

49. + Critical Weathermaker Stable Air: A parcel of air that is cooler than its surrounding environment, it would be more dense; and it would sink to its original position Resists vertical movement

50. + Critical Weathermaker Unstable Air: A parcel of air that is warmer than its surrounding environment, it would be less dense; and it would rise until its temperature was the same as its surroundings

51. + Critical Weathermaker Stability is a property of air that describes its tendency to remain in its original position (stable) or to rise (unstable)

52. + Absolute Stability When environmental lapse rate is less than the wet adiabatic Even if this stable air were forced above the condensation level, it would remain cooler and denser than its environment and would have a tendency to return to the surface

53. +

54. + Absolute Stability Temperature Inversion: the most stable conditions occur when the temperature in a layer of air actually increases with altitude

55. + Absolute Instability The environmental lapse rate is greater than the dry adiabatic rate Occurs most often during the warmest months and on clear days when solar heating is intense

56. +

57. + Conditional Instability Moist air has an environmental lapse rate between the dry and wet adiabatic rate The atmosphere is conditionally unstable when it is stable with respect to an unsaturated parcel of air, but unstable with respect to a saturated parcel of air Depends on whether or not the rising air is saturated

58. +

59. + Daily Weather When stable air is forced aloft, the clouds that form are widespread and have little vertical thickness Light to moderate precipitation Clouds associated with unstable air are towering and are usually accompanied by heavy precipitation

60. + Daily Weather Any factor that causes air near the surface to become warmed in relation to the air aloft increases the air’s instability Any factor that causes the surface air to be chilled results in the air becoming more stable