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 km 3 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 of total water Oceans97.24% Ice caps, glaciers2.14% Ground water0.61% Fresh-water lakes0.009% Inland seas0.008% Soil moisture0.005% Atmosphere<0.001% Rivers <0.0001% Total water volume100% Source: U.S. Geological Survey

6. The Unique Properties of H 2 O 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 H 2 O Ice takes up as much as 9% more space than the same number of liquid H 2 0 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 H 2 O Pure water is most dense at 4  C Water expands above or below that temperature Fills its container, but non-compressible H 2 0 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 = [H 2 0 vapour content/H 2 0 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 1.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)

14. 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: 1.Hair hygrometer 2.Sling psychrometer

15. 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

16. 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

17. 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”

18. 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

19. 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

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

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

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

23. Cloud Formation Air rises to altitude where RH=100% H 2 O vap  H2O liq 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

24. 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

25. 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