Moisture and Atmospheric Stability


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Moisture and Atmospheric Stability

  1. 1. +Moisture and Atmospheric StabilityChapter 4
  2. 2. +4.1 Movement of Water Through theAtmosphere
  3. 3. + Movement of Water Through The Atmosphere 97% of all water is salt water Remaining 3% is stored in ice sheets in Antarctica and Greenland Only 0.0001% is found in the Atmosphere in the form of water vapor
  4. 4. + Movement of Water Through The AtmosphereHydrologic Cycle:  Thecontinuous exchange of water among oceans, atmosphere and continents
  5. 5. +4.2 Water: A Unique Substance
  6. 6. + Water: A Unique Substance Water: 1. Water is the only liquid on the surface of the Earth in large quantities. 2. It exists in all forms on Earth. 3. Ice (solid state) is less dense than liquid. 4. Water has a high heat capacity.
  7. 7. + Water: A Unique Substance Ithas a unique ability to form hydrogen bonds (H2O) When ice forms on a body of water it insulates the underlying layer of water and slows the rate of freezing  Thisis how fish can survive in a lake during the winter :)
  8. 8. +4.3 Water’s Changes of State
  9. 9. + Water’s Changes of State Ice, liquid water, and water vapor:
  10. 10. + Water Changes States Ice  Tightorderly network of molecules vibrating in a fixed state Water  Molecules can slide past one another Gas  Very random erratic motion
  11. 11. + Latent Heat  Whenever water changes state heat is lost to its surroundings. Latent heat is hidden heat  Adding heat to melt ice does not result in a temperature change.  Melting 1 gram of ice requires 80 calories.  Latent heat of melting  Freezing 1 gram of water releases 80 calories.  Latent heat of fusion
  12. 12. + Latent Heat Evaporation  The process of converting a liquid to a gas.  The latent heat of vaporization is the energy absorbed by water during evaporation.  ~ 600 calories/gram for water  Evaporation is a cooling process.
  13. 13. + Latent Heat Condensation  Is the reverse process, converting a gas to a liquid.  The process when water vapor changes to the liquid state is the latent heat of condensation.  Energy is released, which warms the surrounding air.  Forms clouds and fog
  14. 14. + Latent Heat Sublimation:  Sublimationis the process that turns a solid to a gas.  Disappearing ice cubes in freezer are a result of sublimation. Deposition:  Deposition is the reverse process of changing a vapor to a solid.  Frost accumulating in a freezer is a result of deposition.
  15. 15. +4.4 Humidity: Water Vapor in the Air
  16. 16. +Humidity: Water Vapor in the Air Humidity is amount of water vapor in the air. 1. Absolute humidity is the mass of water vapor in a given volume of air. 2. The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air.  Meteorologists like to use this one because it is not affected by changes in pressure or temperature
  17. 17. + Humidity: Water Vapor in the Air 3. Vapor pressure is defined as the part of the total atmospheric pressure attributable to its water-vapor content. 4. Relative humidity indicates how close air is to saturation rather than the actual quantity of water vapor in the air. 5. Dew point is the temperature to which air needs to be cooled to reach saturation.  **We will talk more about these 3
  18. 18. +4.5 Vapor Pressure and Saturation
  19. 19. + Vapor Pressure and Saturation Vapor pressure is that part of the total air pressure attributable to water vapor content.  Morewater vapor equals more vapor pressure
  20. 20. + Vapor Pressure and Saturation Saturation:  Saturationis the equilibrium point between evaporation and condensation.  Temperature dependent  For every 10°C (18 °C) increases in temperature, the amount of water for vapor needed for saturation doubles  Humid air equals a high vapor pressure.
  21. 21. +4.6 Relative Humidity
  22. 22. + Relative Humidity Relative humidity:  Relative humidity is the ratio of the air’s actual water vapor content and amount of water vapor required for saturation at a certain temperature and pressure.  Relative humidity indicates how near the air is to saturation rather than the actual quantity of water vapor in the air.
  23. 23. + Relative Humidity At25°C, air is saturated at 20 g/kg Soif we had 10g/kg at 25°C then the relative humidity is 10/20 or 50%
  24. 24. + Relative Humidity How relative humidity changes:  100% relative humidity equals saturation.  If water vapor is added, relative humidity goes up.  If water vapor is removed, relative humidity goes down.  A decrease in temperature equals an increase in relative humidity.
  25. 25. + Relative Humidity  A decreasein temperature equals an increase in relative humidity.  When air aloft is cooled below its saturation level, some of the water vapor condenses to form clouds. Clouds are made of liquid droplets, so the moisture is no longer apart of the water- vapor content of the air.
  26. 26. + Relative Humidity
  27. 27. + Relative Humidity Natural changes:  Daily temperature changes affect relative humidity.  Temperature changes are caused by advection, the primarily horizontal component of convective flow (wind).  Temperature changes are also caused through convection, where some of the air in the lowest layer of the atmosphere, heated by radiation and conduction, is transported by convection to higher layers of the atmosphere.
  28. 28. +4.7 Dew-Point Temperature
  29. 29. + Dew-Point Temperature Dew point:  The dew point is the temperature air is cooled to reach saturation.  Dew-point temperature is a measure of actual moisture content.
  30. 30. + Dew-Point Temperature In nature, cooling below the dew point cause water vapor to condense, typically as dew, fog or clouds
  31. 31. +4.8 How is Humidity Measured?
  32. 32. + How is Humidity Measured?  Hygrometer:  A hygrometer measures moisture the content of air.  A hair hygrometer operates on the principle that hair changes length in proportion to changes in relative humidity.  These are harder to use and need to be calibrated often
  33. 33. + How is Humidity Measured? A psychrometer consists of two identical thermometers; one (dry thermometer) measures air temperature, the other called the “wet bulb,” measures the dryness of air.  Thelarger the difference in temperatures between the wet and dry, the lower the relative humidity (with a table).
  34. 34. +4.9 Adiabatic Temperature Changes
  35. 35. + Adiabatic Temperature Changes Remember condensation occurs when sufficient water vapor is added to the air or more commonly when the air is cooled to its dew point temperature. An adiabatic process is one in which no heat is transferred.  When air is compressed, it warms.  When air expands, it cools.
  36. 36. + Adiabatic Temperature Changes Adiabatic cooling:  Cooling occurs when air moves up and it expands and cools.  Unsaturated air cools at a rate of 10° C/1000m; this is called the dry adiabatic rate.
  37. 37. + Adiabatic Temperature Changes  Adiabatic condensation:  Condensation is triggered when air rises high enough to reach its saturation point and clouds form, called its lifting condensation level.  When air ascends above the lifting condensation level, the rate at which it cools is reduced. The slower rate of cooling is called the wet adiabatic rate (because the air is saturated), which varies from 5° C/1000m.
  38. 38. + Adiabatic Temperature Changes
  39. 39. + Adiabatic Temperature Changes
  40. 40. +4.10 Processes That Lift Air
  41. 41. + Processes That Lift Air 4 mechanisms that cause to air to rise 1. Orographic lifting in which air is forced to rise over a mountainous barrier 2. Frontal wedging in which warmer less dense air is forced over cooler dense air 3. Convergence is a pileup of horizontal air flow that results in upward motion 4. Localized convective lifting is unequal surface heating causes localized pockets of air to rise because of their buoyancy.
  42. 42. + Processes That Lift Air Orographic lifting occurs when elevated terrains, such as mountains, act as barriers to the flow of air.  Adiabatic cooling can generate clouds and copious precipitation. Many of the wettest places in the world are located on windward mountain slopes.  When air reaches the leeward side, much of its moisture has been lost.
  43. 43. + Processes That Lift AirRain shadow desert- happens on leeward mountain side, adiabatic warming making ran less likely
  44. 44. + Processes That Lift AirFrontal wedging:  Masses of warm and cold air collide, producing fronts. Cooler, denser air acts as a barrier over which the warmer, less dense air rises. This process is called frontal wedging.
  45. 45. + Processes That Lift Air Convergence:  Converging horizontal air flow results in upward movement.  Can also happen when an obstacle slows or restricts horizontal air flow
  46. 46. + Processes That Lift Air Localized convective lifting:  Unequal surface heating causes localized pockets of air to rise.  Can rise high enough, condense and form clouds
  47. 47. +4.11 The Critical Weather maker:Atmospheric Stability
  48. 48. +The Critical Weathermaker:Atmospheric Stability Stable air resists vertical movement. Unstable air rises due to buoyancy till it reaches a temperature that is equal to its surroundings. Environmental lapse rate is the actual temperature of the atmosphere.  Airtemperature is measured at various heights in the atmosphere.
  49. 49. + The Critical Weathermaker: Atmospheric Stability Absolute Stability  The environmental lapse rate is less than the adiabatic rate  The rising air continues to cool Temperature inversion  Type of environmental lapse rate when the temp in a layer of air increases with altitude rather than decreasing
  50. 50. +The Critical Weathermaker:Atmospheric Stability
  51. 51. + The Critical Weathermaker: Atmospheric Stability Absolute Instability  The environmental lapse rate is greater than the adiabatic rate  The ascending parcel of air is always warmer than its environment and will continue to rise because of its buoyancy  Occurs most often during the warmest months on clear days when solar heating is intense
  52. 52. +The Critical Weathermaker:Atmospheric Stability
  53. 53. +The Critical Weathermaker:Atmospheric StabilityConditional instability is the most common type of atmospheric instability.  This situation prevails when moist air has an environmental lapse rate between the dry and wet adiabatic rates.
  54. 54. + The Critical Weathermaker: Atmospheric Stability
  55. 55. + In Summary: Atmospheric Stability The stability of air is determined by measuring temperature of the atmosphere at various temperatures (environmental lapse rate) A column of air is unstable when the air near the bottom of this layer is significantly warmer than the air aloft
  56. 56. + In Summary: Atmospheric Stability Air is considered stable when the temperature decreases gradually with an increase in altitude Most stable condition occurs during a temperature inversion when the temperature actually increases with height.  Under these conditions there is little vertical air movement
  57. 57. +4.12 Stability and Daily Weather
  58. 58. + Stability and Daily Weather On a dreary overcast day with light drizzle, stable air is forced aloft On a day with towering clouds forming the atmosphere is unstable
  59. 59. + Stability and Daily Weather How stability changes:  Instability is enhanced by the following:  Intense warming of the lowest layer of the atmosphere  Heating of an air mass from below  General upward movement of air caused by orographic lifting, frontal wedging, and convergence  Radiation cooling from cloud tops
  60. 60. + Stability and Daily Weather How stability changes:  Stabilityis enhanced by the following:  Radiation cooling of Earth’s surface after sunset  Cooling of an air mass from below as it traverses cold surface  General subsidence within an air column
  61. 61. + Stability and Daily Weather Temperature changes and stability:  As air moves horizontally over the surface with different temperatures changes in stability occur  Ex: When warm air from the Gulf of Mexico moves north over the snow covered Midwest, the air is cooled from below, it becomes more stable, often producing widespread fog.
  62. 62. + Stability and Daily Weather  Ex:In winter, polar air move south over over the Great Lakes. Polar air is then rendered sufficiently unstable when the cold, dry air passes over a warm, wet surface, which can often produce lake effect snow over the Great Lakes.
  63. 63. +Vertical Air Movement and StabilitySubsidence is a general, downward air flow.Usually, surface air is not involved.When there is a general downward airflow, the upper portion of the subsiding layer is heated by compression more than the lower. So, the net effect is to stabilize the air because the air aloft is warmed more than the surface air.  Thewarming effect of a few 100m of subsidence is enough to evaporate the clouds. Resulting in clear blue skies :)
  64. 64. + Final Summary The air’s stability or, lack of it, determines to a large degree whether clouds develop and produce precipitation and whether that precipitation will come as a gentle shower or a violent down pour.
  65. 65. + Final Summary In general, when stable air is forced up, the associated clouds have very little vertical thickness and precipitation, if any, is light. In contrast, clouds associated with unstable air are towering and are frequently accompanied by heavy precipitation.