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Atmospheric Water

Atmospheric Water. and Weather. A Brief History of Water Outgassing Water on Earth formed within the planet Massive quantities outgassed into early atmosphere Torrential rains created lakes and oceans Flows of water over land carried dissolved and undissolved elements to oceans

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Atmospheric Water

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  1. Atmospheric Water and Weather

  2. A Brief History of Water • Outgassing • Water on Earth formed within the planet • Massive quantities outgassed into early atmosphere • Torrential rains created lakes and oceans • Flows of water over land carried dissolved and undissolved elements to oceans • Present volume of water 1.36 billion km3 reached about 2 billion years ago • Volume of water is quite stable (loss to space/compounds equalled by supply from below)

  3. Some Simple Facts about Water 71 % of Earth's surface is water (by area) The weight of water is 1kg/L Sea Levels Eustatic sea level change is controlled by: water temperature and ice sheet/glacier volume Mean sea level is currently rising (interglacial) Sea level was 100m lower 18,000 BP

  4. Distribution of Water on Earth • 97.2% of all surface water is oceanic • 2.8% is non-oceanic • Most of Earth's freshwater is frozen in ice sheets/glaciers • Rest is in lakes, rivers, groundwater or soil moisture

  5. Water source Percent oftotal water Oceans 97.24% Ice caps, glaciers 2.14% Ground water 0.61% Fresh-water lakes 0.009% Inland seas 0.008% Soil moisture 0.005% Atmosphere <0.001% Rivers <0.0001% Total water volume 100% Source: U.S. Geological Survey

  6. The Unique Properties of H2O • 1.A Solvent • Water molecules attracted to one another •  side (2H) attracted to - side (O) of another molecule • H-bonds form between molecules - cause of surface tension and capillarity • 2.Heat Properties • Three phases - solid, liquid, vapour

  7. Phase changes Melting: Solid  Liquid Freezing: Liquid  Solid Evaporation/Vaporization: LiquidVapour Condensation: Vapour Liquid Sublimation: SolidVapour Deposition: VapourSolid

  8. Frozen H2O • Ice takes up as much as 9% more space than the same number of liquid H20 molecules • Ice floats because it weighs only 91% as much as water • To melt, heat energy must increase molecular motion until H-bonds break • Latent heat of fusion is large compared to heat necessary to heat ice or water without a phase change An iceberg is 91% below water surface

  9. Liquid H2O • Pure water is most dense at 4C • Water expands above or below that temperature • Fills its container, but non-compressible • H20 Vapour • Water that evaporates must absorb energy – • latent heat of evaporation • The dominant cooling process in the Earth's energy budget • Water vapour that condenses liberates energy • latent heat of condensation

  10. Humidity • Water vapour content of air is its humidity • Warm air holds more water as vapour than cold air • Relative humidity: A ratio that compares the amount of water vapour in the air to the maximum water vapour capacity at that temperature • The relative humidity of saturated air is 100% • RH = [H20 vapour content/H20 capacity] x 100

  11. What affects relative humidity? 1. temperature changes 2. evaporation 3. condensation 4. advection At saturation, any decrease in temperature or addition of water vapour results in condensation Dew point temperature: the temperature at which air becomes saturated When RH = 100%, the air temperature and the dew point temperature are the same RH is highest at dawn and lowest in the afternoon (warmer).

  12. How to Express Humidity • Vapour pressure: • the portion of atmospheric pressure that is made up of water vapour molecules (mb or kPa) • water evaporates from a moist surface until the increasing vapour pressure in air causes some molecules to return to the surface • maximum capacity of air to hold moisture referred to as saturation vapour pressure, the maximum pressure that water molecules can exert • Saturation vapour pressure changes with temperature (almost doubles with each 10C rise)

  13. Specific humidity: the mass of water vapour (g) per mass of air (kg) Maximum specific humidity is the maximum mass of water vapour that can be held by 1kg of air at a given temperature • Humidity Measurements: • Hair hygrometer • Sling psychrometer

  14. Sling psychrometer • Wet-bulb/Dry bulb thermometers • wet bulb thermometer has its bulb moistened with a wick and air is passed over it • the temperature depression is determined by dryness • temperatures the same when relative humidity = 100% • wet bulb measures a much lower temperature if the air is dry (due to evaporation) • Psychrometric chart is required

  15. Atmospheric Stability A 'parcel of air' is a body of air that has particular temperature and humidity characteristics. Warm air has a lower density Cold air has a higher density A parcel of lower density air will rise and expand as external pressure decreases A parcel of higher density air will descend and be compressed by higher external pressure

  16. Stability • The tendency of a parcel to remain in place or change vertical position by ascending or descending • To measure stability we need to understand the temperature distribution at a range of heights • Measured with an instrument package called a radiosonde • Normal lapse rate: 6.4C/km • Environmental lapse rate: ?.? C/km • In the absence of external heating and cooling… • Ascending air cools with expansion • Descending air heats due to compression “adiabatic”

  17. Dry adiabatic lapse rate: The rate at which dry air cools by expansion or warms by compression with a change in height. DALR = 10C/1000m Moist adiabatic lapse rate: The rate at which moist ascending air cools by expansion MALR typically about 6C/1000m Varies: 4C/1000m in warm air near 10C/1000m in cold air Latent heat of condensation liberated as parcel rises

  18. Unstable conditions ELR > DALR Rising parcel of air remains warmer and less dense than surrounding atmosphere Stable conditions ELR < MALR Rising parcel of air becomes cooler and denser than surrounding air, eliminating the upward movement Conditionally unstable conditions DALR>ELR>MALR

  19. Lifted parcel is theoretically cooler than air after lifting ELR =  DALR =  Source: http://www.atmos.ucla.edu

  20. Lifted parcel is theoretically warmer than air after lifting ELR =  DALR = 

  21. Lifted parcel is the same temperature as air after lifting Note: Conditionally-unstable conditions occur for m <  < d

  22. Cloud Formation • Air rises to altitude where RH=100% • H2Ovap H2Oliq on condensation nuclei • Cloud Types • Stratiform - layered • Cumuliform - globular or puffy • Cirroform – wispy, always composed of ice • Rain clouds: nimbostatus (light), cumulonimbus (heavy) • Mid-level clouds: altostratus, altocumulus • High-level clouds: cirrus, cirrostratus, cirrocumulus

  23. Fog Ground-level cloud Visibility less than one kilometre Advection fog 1. Warm, moist air passes over cooler surface 2. Cold air flows over warm body of water (evaporation or steam fog) 3. Upslope fog (hills force moist air upward) 4. Valley fog (cool air settles into low-lying areas) Radiation fog Radiational cooling on clear nights brings air temperature to dew point near the ground

  24. Air Masses Continental Polar – cP Maritime Polar – mP Continental Tropical – cT Maritime Tropical – mT Atmospheric Lifting Mechanisms Convectional lifting Convergence lifting Orographic lifting Review: Cold fronts, warm fronts and mid-latitude cyclones

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