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Atmospheric Moisture






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Atmospheric Moisture Atmospheric Moisture Presentation Transcript

  • Water and Moisture in the Atmosphere
  • Overview
    • Water
      • Distribution
      • Properties
    • Humidity
      • Relative Humidity
      • Specific Humidity
    • Atmospheric Stability
      • Normal and Environmental Lapse Rates
      • Dry and Moist Adiabatic Rates
      • Stability and Instability in the Atmosphere
    • Clouds and Fog
  • Global Distribution of Water
    • Sources
        • Outgassing
        • Transpiration
        • Combustion and related processes (respiration, decomposition)
    • Distribution
      • Ocean and Land Hemispheres
      • Eustasy
      • Percentages
        • Oceans: 97% (Pacific – 50%, Atlantic – 26%, Indian – 20%, Arctic – 4%)
        • Freshwater: 3%
          • Surface water: 2% (1.99% as ice and glaciers)
          • Groundwater: 0.6%
          • Soil Moisture: < 0.5%
  • Properties of Water
    • Polar Structure
      • Chemical structure gives H 2 O molecules an electrostatic polarity
        • Unequal sharing of electrons
          • Oxygen has a higher electronegativity than Hydrogen
          • Covalent bonds
        • Asymmetrical (bent) shape
        • Positive charge associated H, Negative with O
      • Hydrogen bonding
      • Cohesive force, surface tension
      • Powerful solvent
      • Affects soil water availability, nutrient transport
  • Oxygen H H  +  -
    • Heat Properties
      • Heat exchanges with water accounts for 30% of energy driving atmospheric circulation
      • Phases
        • Solid – Ice (also snow, sleet, hail, frost)
          • Maximum density at 4C o , Density of ice is 0.9 that of water
          • Hydrogen bonding creates hexagonal crystal structure
        • Liquid – water (rain, dew)
          • Hydrogen bonds are fewer, create surface tension, cohesion
        • Gas – water vapor (steam, clouds, humidity)
      • Phase changes
        • Latent Heat – amount of heat released/absorbed in a phase change
          • Latent Heat of Freezing/Melting
          • Latent Heat of Evaporation/Condensation
        • Freezing-melting
        • Condensation-evaporation/vaporization (precipitation, dew)
          • The primary means of heat exchange with the environment
        • Deposition-sublimation (frost)
  • Temp . Heat Specific Heat Latent Heat Latent Heat Specific Heat Specific Heat Ice Water Vapor 80 Cal 100 Cal 540 Cal 20 Cal
  • Humidity
    • Humidity is water content of the air
      • Amount of water is directly proportional to temperature
    • Relative Humidity is the ratio of water content in the air to its capacity, expressed as a percentage
      • Capacity varies with temperature over the daily range of temperatures
      • Content varies with the movement of air masses, annual pattern of pressure and oscillations
    • Saturation
      • Occurs when relative humidity becomes 100%
      • Most often occurs when capacity is reduced due to a drop in temperature
      • Further increases in content/reduction of capacity results in condensation
        • Clouds, fog, precipitation
    • Dew-point
      • The temperature at which air becomes saturated
    • Vapor Pressure
      • Water vapor is a gas, so it contributes to atmospheric pressure
      • Vapor pressure refers to the amount of pressure exerted by water vapor alone
        • Expressed in mb
        • Varies with temperature
      • Saturation occurs when water molecules entering the air are in equilibrium with those leaving it
        • Saturation vapor pressure
      • The percentage ratio of VP to SVP gives relative humidity
  • At 30 o C, if the vapor pressure is 15mb, then the relative humidity is 33.3% 15mb/45mb (SVP) × 100
    • Specific Humidity
      • Humidity expressed as actual mass of water in the air per mass of air (g/kg)
      • Maximum specific humidity is the maximum mass of water vapor that the air can hold at any given temperature
      • Relative humidity is the ratio of SH to MSH expressed as a percentage
  • At 20 o C, if the specific humidity is 5g, then the relative humidity is 33.3% 5g/15g (MSH) × 100
  • Atmospheric Stability
    • Storms are caused by atmospheric instability
      • Instability is rising air that can produce cyclonic rotation
      • Stability occurs when air masses are not rising
    • The relative stability of the atmosphere is the product of two cooling processes
      • Environmental Lapse Rate
        • The rate at which air cools with altitude
        • As measured at any given time.
        • The flow of heat energy through the atmosphere greatly affects this rate
      • Adiabatic Rate
        • The rate at which air cools as it becomes less dense
        • Based on thermodynamic laws of gases relating temperature to density and pressure – it is the rate of cooling simply by expanding, without the flow of heat energy into or out if it.
    • If the adiabatic rate exceeds the environmental rate, then air will cool more quickly by rising than the surrounding air, and will not rise
    • If the adiabatic rate is less than the environmental lapse rate, then rising air will have a higher temperature than the surrounding air, and will continue to rise
    • Two adiabatic rates:
      • Dry adiabatic rate (DAR) if air is not saturated
        • 10 o C per 1000m
      • Moist adiabatic rate (MAR) if it is saturated
        • 6 o C per 1000m
      • If a parcel of air rises far enough, it will cool, and its capacity for water vapor will decrease, and become saturated. If so, the adiabatic rate for the parcel changes from DAR to MAR
  • Clouds and Fog
    • An expression of stability in the atmosphere
    • Formation
      • Air at some point in the atmosphere becomes saturated
      • Water droplets condense around particles of dust, soot, pollution or sea-salt
        • condensation nuclei
    • Classification
      • Based and altitude and shape
      • Forms
        • Stratiform: flat and layered
        • Cumuliform: puffy, globular and vertically developed
        • Cirroform: Wispy clouds (typicaly high altitude)
    • Cloud Types
      • Low clouds (up to 2000 m)
        • Stratus: Dull, gray and featureless
        • Stratocumulus: puffy stratiform clouds
        • Nimbostratus: Stratus clouds with drizzling precipation
      • Middle clouds (2000 – 6000 m)
        • Altostratus: High, thin stratiform clouds. Sun just visible
        • Altocumulus: Patches of “cotton balls” arranged in lines, groups, rippling waves
      • High clouds (6000 – 13,000 m)
        • Cirrus: wispy, feathery, hairlike, streaks or plumes
        • Cirrostratus: Milky veil of ice crystals – sun and moon halos
        • Cirrocumulus: Tufts, dappled, in lines, groups or ripples
      • Vertically developed clouds (0 – 13,000 m)
        • Cumulus: Flat based puffy tops; sharply outlined; fair weather
        • Cumulonimbus: Cirrus-topped plume blown by upper level winds into an anvil-shape; explosive tops; extreme vertical development; thunderstorms, tornadoes.
    • Fog
      • Stratus clouds sitting on the ground
      • Advection Fog
        • Warm air moving over a cool surface, becoming saturated
        • Cool air moving over a warm surface, saturating it
          • evaporation fog
        • Warm moist air being forced upslope, cooling and becoming saturated
          • Upslope fog
        • Warm layer of air moving over trapped lower lying cool, saturated air, creating an inversion layer
          • Valley Fog
      • Radiation Fog
        • Radiative cooling of a surface saturates the air above it