2. Moisture and Precipitation

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					2. Moisture and
 Precipitation
   14 March 2010
                Lecture Outline

 Moisture in the atmosphere

 Initiation of clouds: growth by condensation

 Growth in warm clouds

 Growth in cold clouds

 Forms of precipitation

 Mist, fog, smog, dew and frost

 Weather modification
     Moisture in the Atmosphere
•   Water is fundamental to all life forms on earth

•   Precipitation is necessary for the removal of
    moisture from the atmosphere and onto the
    earth’s surface

•   Before precipitation can take place:
    - condensation must occur
    - clouds must form
    - large enough water drops / ice crystals must
    develop to fall out of clouds
Water is known to exist in three different states; as a
                solid, liquid or gas.
            INFLUENCE OF WATER VAPOUR ON
                 ATMOSPHERIC PROCESSES
•       Water vapour important in formation of precipitation

•       Moisture content of atmosphere influences the degree of
        stability of the atmosphere

•       Also plays a key role in atmospheric heat budget
    -     Reflects incoming solar radiation back to space
    -     Scatters incoming solar radiation
    -     Reflects long-wave terrestrial radiation back to earth
    -     Absorbs outgoing terrestrial radiation (GHG)
    -     Latent heat release
               PRECIPITATION
• Clouds, snow, and rain are all made of up of
  H2O:
  – A cloud is comprised of tiny water droplets
    and/or ice crystals
  – A snowflake is an aggregate of ice crystals
    (crystalised water)
  – Raindrops are liquid water droplets
       Initiation of Clouds: Growth by
                Condensation
• Water molecules are constantly escaping from
  various sources (ocean, rivers, lakes, swamps, the
  soil) – this is known as evaporation.

• The gaseous water molecules in the air are known as
  water vapour
   – These molecules absorb latent heat at evaporation and




                                                             D. Trotter, Discipline of Geography, SES, UKZN
     release it at condensation

• Rate of evaporation is directly proportional to
  temperature – the higher the temperature the
  greater the rate of evaporation.
                  Condensation Process
•       When air is cooled, its ability to ‘hold’ water vapour
       is reduced

•      If cooled sufficiently then a temperature will be
       reached at which the air cannot hold the water
       vapour it contains.
    • The air is said to be saturated at this point
    • The water vapour will condense back into water
    • The temperature at which this takes place is the dew-point
    temperature.

•      The dew-point temperature is directly related to the
       relative humidity
    • The greater the quantity of water vapour in the air, the less the
    air needs to be cooled before it becomes saturated
                  Condensation and Uplift
•In order for clouds to form, it is necessary to cool the air

•In the free atmosphere, most effective mechanism for cooling
is uplift.

• Air can move upwards in four main ways:
       -     Uplift due to convergence (LP cells)
       -     Convectional uplift
       -     Forced uplift of moist moving air encountering
             mountains (orographic) and
       -     Forced uplift of air at the edges of colliding air
             masses associated with cyclonic storms (frontal
             uplift)
                  Condensation & Uplift
• Air moving upward will be chilled by the
  adiabatic process to saturation point.
  – Uplift causes a reduction of pressure in the rising air, air
    expands to compensate for the reduction of pressure and
    cooling results.

• Condensation will occur.

• Eventually precipitation will form.

• But what about the processes between condensation
  and precipitation?
  • To begin with, let’s look at the role of aerosols in precipitation
    processes...
              Aerosol Characteristics

• If particle-free air is rapidly cooled & RH increases –
  water droplets form by spontaneous nucleation

• In reality, the atmosphere contains airborne aerosol
  particles

• Cloud droplet formation is enhanced through the
  action of these aerosols
  – In effect, they raise the dew-point temperature, at which
    condensation will occur (i.e. Lower the amount of
    cooling required for condensation to occur)
               Aerosol Characteristics
• In the atmosphere, water vapour condenses on tiny
  specks of material suspended in the air, known as
  aerosols

• These condensation nuclei broadly grouped into 3 size
  categories:
  - Aitken nuclei (diameter less than 0.2 µm) -
      combustion particles, products of chemical reactions
  -   large aerosols (between 0.2 & 2 µm) –




                                                             D. Trotter, Discipline of Geography, SES, UKZN
      ash, sea salt, pollen, coagulation of above
  -   giant aerosols (larger than 2 µm) -
      windblown dust, industrial emissions
                                      Growth of cloud droplet
(Source: http://earthsci.org/processes/weather/weaimages/ccprcess.gif   )

   • Initially condensation occurs around these condensation nuclei
     (aerosols)

   • Microscopic cloud droplets (10 - 20µm radius) are formed
     around each condensation nuclei




                                                                            D. Trotter, Discipline of Geography, SES, UKZN
   • Rate of growth of a droplet by condensation is rapid while
     droplet is small, slows as droplet size increases
      • Condensation alone cannot account for the development of
        raindrops

      • Other processes are required to produce the rapid growth of
        clouds
(Source: Vega, undated. Understanding Weather and Climate. Chp 7. taken from: http://twister.sbs.ohio-state.edu/g520/ch7_1.pdf)
      Other mechanisms are required to make drops
       grow larger
      • Growth in warm clouds: collision-coalescence and
        wake capture
      • Growth in cool and cold clouds: Bergeron-Findeison
        Theory




                                                                                                                                  D. Trotter, Discipline of Geography, SES, UKZN
                           Cold Clouds                                                                         Warm Clouds

(Source: Vega, undated. Understanding Weather and Climate. Chp 7. Taken from: http://twister.sbs.ohio-state.edu/g520/ch7_1.pdf)
          Growth in Warm-clouds
  Collision-coalescence & wake capture
• Some liquid droplets larger than others because of the varying
  size of condensation nuclei

• As they fall, larger droplets overtake their neighbours, and
  collide and join with smaller ones - coalescence

• In the process, droplet size increases rapidly




                                                                   D. Trotter, Discipline of Geography, SES, UKZN
• Small droplets may also be pulled into the wake of the larger
  drop by wake capture
Collision-coalescence & wake capture




                                         D. Trotter, Discipline of Geography, SES, UKZN
  (Source:Tyson & Preston-Whyte, 2000)
   Collision-coalescence & wake capture
• Once started, process accelerates and droplet grows
  rapidly by coalescence when growth by condensation
  is slow

• Process of collision and coalescence occurs in warm
  clouds (ambient temp is greater than 0° Celcius) and
  condensation particles consist of liquid water

• Updraughts will keep cloud particles in suspension for




                                                           D. Trotter, Discipline of Geography, SES, UKZN
  as long as they exceed the gravitational force
  downwards

• When the gravitation forces exceeds updraught force –
  droplets fall out of clouds as rain
                 Terminal Velocity

• Terminal velocity is achieved when gravity
  and frictional drag eventually become equal as
  the drop falls to earth
                                                     Fgravity
• Due to their small size, the downward gravity
  force for cloud drops does not exceed even
  weak updraughts




                                                                D. Trotter, Discipline of Geography, SES, UKZN
• In order for precipitation to be generated, the   Fdrag

  volume of a cloud drop is typically 1,000,000x
  greater than average drop to overcome
  updraughts.
                                             growth                                      Time passes
                                                                                                                      Rain
                   Cloud                                           Rain                                               Droplet
                   Droplet                                         Droplet                                            @ terminal velocity



                                                                                  Fgravity                                        Fgravity
                                 Fgravity




                               Fdrag
                                                                                Fdrag                                             Fdrag




                                                                                                                                             D. Trotter, Discipline of Geography, SES, UKZN
           Fgravity < Fdrag                               Fgravity > Fdrag                                        Fgravity = Fdrag

(Source: Vega, undated. Understanding Weather and Climate. Chp 7. taken from: http://twister.sbs.ohio-state.edu/g520/ch7_1.pdf)
                      Liquid droplets - rain
• Cold-cloud rain
  snow flakes passing through warm air → melt → rain


• Warm-cloud rain
  droplet growth by coalescence


• Rain showers




                                                                              D. Trotter, Discipline of Geography, SES, UKZN
  Some drops reach diameters of 5mm → become unstable & fragment into
  smaller particles
  Carried aloft and coalescence process repeated
  Chain reaction of raindrop multiplication results in shower precipitation
  Associated with convective activity and cumulus clouds
           Growth in cold-clouds
• Clouds with temp lower than 0°C are cold
  clouds

• Temps between 0°C and -40°C, water droplets
  can exist as supercooled droplets

• Usually co-exist with ice crystals – mixed cloud




                                                     D. Trotter, Discipline of Geography, SES, UKZN
• Temps below -40°C only ice crystals occur
                      Ice-crystal Formation
• Ice crystals initially form as a result of various nucleation
  processes:
  - spontaneous nucleation of water droplets
       - temperatures below 40˚C
  - heterogeneous nucleation
        - a droplet on a freezing nucleus
        - generally at temperatures > 40˚C
        - the larger the droplet, the lower the temperature required for freezing
        - most efficient nuclei have a crystallographic structure similar to ice
  - contact nucleation
       - mere contact of freezing nucleus lowers temperature sufficiently for nucleation
         onto the nucleus
  - sublimation
        - water vapour  ice on deposition nuclei – particularly effective when
          temperatures are low
        - at -10˚C, air saturated with respect to water is supersaturated with respect to ice
          by 10%
        - at -20˚C, by 20%
              Ice-crystal Growth
•   In cold layer-type clouds, ice crystals grow
    into snowflakes
•   In deep convective clouds they develop
    as hailstones
•   Ice crystals may be of different size and
    shapes
•




                                                   D. Trotter, Discipline of Geography, SES, UKZN
    May grow by sublimation and collision
    (much the same way as growth of liquid
    drops by condensation & coalescence)
                     Ice-crystal Growth
• Sublimation
  - transition from vapour to ice on nuclei (freezing, contact, deposition)
  due to high supersaturations (see Bergeron-Findeison Theory)

• Collisions with ice crystals
  - ice crystals grow by collision with other ice crystals
  - known as growth by aggregation (become entangled &
  entwined)
  - most effective at temperatures around -5˚C




                                                                              D. Trotter, Discipline of Geography, SES, UKZN
• Collisions with supercooled droplets
   - Mixed clouds
   - Leads to growth by riming (supercooled liquid droplets freeze onto
     the ice crystal)
   - Graupel  hail
                 Snowflakes

• Snow flakes are of variable shapes and sizes
  (depends on temperature and moisture
  content)




                                                 D. Trotter, Discipline of Geography, SES, UKZN
                     Ice-crystal Growth
• Sublimation
  - transition from vapour to ice on nuclei (freezing, contact, deposition)
  due to high supersaturations (see Bergeron-Findeison Theory)

• Collisions with ice crystals
  - ice crystals grow by collision with other ice crystals
  - known as growth by aggregation (become entangled &
  entwined)
  - most effective at temperatures around -5˚C




                                                                              D. Trotter, Discipline of Geography, SES, UKZN
• Collisions with supercooled droplets
   - Mixed clouds
   - Leads to growth by riming (supercooled liquid droplets freeze onto
     the ice crystal)
   - Graupel  hail
             Ice particles - graupel

• Ice crystals that undergo extensive riming
• Such riming obscure the original shape of the ice
  crystal
• Either fall to the ground or provides a nucleus for
  hail
• Extreme form of this type of growth is the
  hailstone
              Ice particles - hail
                                             (Photo: unknown)

• In many clouds, there are strong updraughts and
  downdraughts of air
• Water droplets or ice crystals may be caught in these
  up- and downdraughts
• When they rise, the water droplets may freeze or ice
  crystals may grow in size
• On each successive trip upwards, ice particles may
  collect more ice until they are too heavy for the
  updraught and fall to earth as hail
• Growth history of the hail stone as it falls and rises is
  reflected in the alternate clear and opaque layers
      Bergeron-Findeison Theory

• Saturation vapor pressure of ice < super-cooled
  water

• When ice and water are present, water will be
  deposited directly onto ice




                                                       D. Trotter, Discipline of Geography, SES, UKZN
• Ice crystals grow rapidly at the expense of super-
  cooled drops
                        Bergeron-Findeison Theory




                                                                                                                                  D. Trotter, Discipline of Geography, SES, UKZN
(Source: Vega, undated. Understanding Weather and Climate. Chp 7. taken from: http://twister.sbs.ohio-state.edu/g520/ch7_1.pdf)
           Bergeron-Findeison Theory
• According to Bergeron-Findeison theory, precipitation
  from cold clouds relies on growth of ice crystals by
  aggregation (collision with other ice crystals) – to form
  snow flakes

• When heavy enough - drops out the cloud as a
  snowflake (when temp between base of cloud and
  ground is below 0°C)




                                                              D. Trotter, Discipline of Geography, SES, UKZN
• If temp in this layer is above 0°C, snow flake melts into
  a water drop and precipitates as rain
                    Summary of Precipitation processes:
                                                               Condensation nuclei




                                         Collision-                                                    Bergeron
                                         coalescence                                                   Process



                                                                                                       Riming/
                                                                                                       Aggregation


                                              Rain                                                        Snow
                                                                                                  (can change to rain, sleet, or
                                                                                                  any other type of
                                                                                                  precipitation depending on
                                                                                                  underlying atmosphere)



(Source: Vega, undated. Understanding Weather and Climate. Chp 7. taken from: http://twister.sbs.ohio-state.edu/g520/ch7_1.pdf)
                   Summary
• Growth of cloud crystals (by sublimation and
  aggregation) and water droplets by (collision
  and coalescence) eventually reach a stage
  where particles can no longer be sustained by
  updraughts

• Precipitation then occurs from the cloud in the




                                                    D. Trotter, Discipline of Geography, SES, UKZN
  form of
  - ice particles (snow, graupel, hail) or
  - liquid droplets (rain)
         Other forms of atmospheric moisture:
               Fog, mist, haze and smog
Fog is ‘thick’ mist
    • Defined according to levels of visibility


• For aviation purposes:
    • Visibility less than 1 km is the result of fog
    • Visibility of less than 2 km but greater than 999.99 m is considered to be
    mist
        •But only if the relative humidity is 95% or greater, below 95%, haze is reported
        – result of dust or smoke
                            Fog & Mist
• Relatively warm, moist air moving over cold ocean water may
result in the formation of fog
       - Sudden drop of temperature of air –
         dew point temperature is reached




• Condensation /cloud formation may occur near the ground
forming mist
       -Sudden drop of temperature of air –
        dew point temperature is reached
       - Over bodies of water or swamps
                    Fog & Mist cont.
- Only a shallow layer of air in
  contact with cold water/
  land surface,
   – A gentle breeze is generally
     necessary to mix cool air with
     air above it


- When fog or mist is mixed
  with pollution, we get smog
   - Often water droplets in smog
     combine with pollutants to form
     corrosive acids
       - SO2 combines with water –
          forms sulphuric acid
                        Dew and Frost
• On calm windless nights, condensation often takes place on cold
surfaces such as blades of grass, roofs of cars and glass
   • Forms dew
   • If air is humid enough, mist / fog may form


• If temps are below freezing point, ice crystals will form
   • Forms frost


• Winds prevent air from remaining in contact with cool
      surfaces for long, thus preventing dew or frost
                     Black / Hoar Frost
• Under certain conditions,
  air may be so dry so that no
  frost forms despite
  temperature having
  dropped to below freezing
  point
   • When this occurs, liquid
     inside the cells of plant
     tissues will freeze, damaging
     the plants cell walls.
   • Parts of the plant turn black
     and often the plant dies – black
     frost
            Weather Modification
• Seeding (pg 37 T&P)– encouragement of precipitation from
  clouds by accelerating droplet formation and enhancing
  efficiency of precipitation-producing mechanisms

• Maritime-type clouds: low drop concentration with a
  relatively broad drop-size distribution

• Continental-type clouds: high drop concentrations with a




                                                                  D. Trotter, Discipline of Geography, SES, UKZN
  narrow drop-size distribution

• Each responds differently to different type of seeding agents
                 Cloud Seeding
-   Stimulate rainfall (warm-cloud with potassium chloride
    and cold-cloud with silver iodide)

-   Suppress hail (artificial ice nuclei – increasing number of
     ice crystals competing for water droplets. Thus
    increasing number of small hailstones and reducing
    number of large damage-causing hailstones)




                                                                  D. Trotter, Discipline of Geography, SES, UKZN
-   dissipate troublesome cold cloud or fog at airports
    (with dry-ice)

				
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