Major Forms of Condensation

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					Major Forms of
Condensation
What are the forms of
  condensation?

 Cloud
 Fog
 Frost and Dew
Differences between Forms of
        Condensation
  Cloud, fog, frost and dew??
   – Cloud           high in the sky
   – fog             near the surface of
     the                        earth
   – frost and dew        at the surface of
     the earth
  Similar in Formation

Cooling (Temperature drops)
Air holds less water
RH increases
When RH = 100%, moisture
released
Frost at 22/12/1999
FROST & DEW
Frost again
Dew at Spider Web
 What is the difference
between Dew and Frost?

–Dew: dew point > 0C
–Frost: dew point  0 C
FOG
Radiation Fog (Conditions)
Clear sky
high RH
temperature inversion
calm and still air
presence of depression or hollow
Advection Fog
Advection Fog
Frontal Fog
Steam Fog
Upslope Fog
CLOUD
        Precipitation
Water droplets from condensation
is very small in size – 0.1 mm
Raindrops are much larger – 0.5-
2.0 mm
The main difference between
condensation and precipitation is
the size of water droplets
   Formation Processes
Air cooling
Condensation and cloud formation
Accumulation of moisture
The growth of cloud droplets
Collision and Coalescence
In tropical and sub-tropical regions
Cloud top temperature :
warmer than -15oC
Turbulence Mixing :
Different size droplets move at different
speeds
cumulonimbus
Collision and Coalescence


                       wake
                       capture
  Fall in   Large
  higher    Water
  speed     droplet

  Fall in             Direct
  lower               capture
  speed
Ice-Crystal (Bergeron) Process
In higher latitudes (temperate regions)
Super-cooled water – water droplets in
cloud with temperature below 0oC
Because of extremely small size of
cloud droplets
Insufficient freezing nuclei
Ice-Crystal (Bergeron) Process
Saturated
vapour
pressure over
super-cooled
water is
greater than
that over ice
Ice-Crystal (Bergeron) Process
Air can be
saturated for
ice when it is
not saturated
for water
Ice-Crystal (Bergeron) Process
Ice crystals
grow at the
expense of
cloud droplets
until they are
large enough
to fall
 3 main types of precipitation
             Convection       Cyclonic / Orographic
               Rain          Frontal rain / relief rain
             Low pressure Warm air          Uplifting
  Lifting    system due to mass meets       along
mechanism    thermal nature cold air mass   slopes

Real world                                  Tai Mo
examples        ITCZ         Polar front
                                            Shan

Diagram of
formation
 Spatial Variation of rainfall
There is more
precipitation in
ocean than in                    ocean
continent.

                         continent
 Spatial Variation of rainfall
Primary
maximum
occurs at a belt                 ocean
between 10oN
to 10oS in the
vicinity of the
                         continent
ITCZ
 Spatial Variation of rainfall
secondary
maximum
occurs at a belt                 ocean
between 40o to
55oN&S in the
vicinity of the
                         continent
ITCZ
 Spatial Variation of rainfall
Primary minima
find in latitudes
higher than
                                 ocean
55oN&S
Secondary
minima in sub-
tropical high            continent
pressure belts
(20o-35oN&S)
 Factors affecting the pattern of
             rainfall
Air pressure
Zones of convergence and divergence
Strong convection along low pressure
belts
Uprising air leads to condensation and
cloud formation
Descending air along high pressure
belts suppress condensation
Describe and
                      50oN
explain the
factor that
influence the
pattern of rainfall    0o
from the
diagram on the
right.
                      50oS
 Zones of abundant rainfall
Around the equatorial regions
 e.g. Amazon Basin
Doldrum low pressure
Convergence of trade winds along
ITCZ
Intense solar radiation promotes strong
convective uplift
Ares dominated by warm moist
equatorial marine air masses
 Zones of abundant rainfall
Western sides in middle latitudes
 e.g. Western Coast of Canada
Under influences of the onshore
westerlies and frequent frontal
cyclones
Western mountrains(Rockies and
Andes) promote orographic influences
and heavy precipitation
Describe and
explain the
factor that
influence the
pattern of rainfall
from the
diagram on the
right.
  Zones of scanty rainfall
Polar areas of North America
Air with low moisture content due to
low temperature
Descending cold upper air masses
unfavourable for condensation
  Zones of scanty rainfall
15o-30oS in the western side of South
America
30o-40oN of south-western U.S.A.
Affected by sub-tropical anticyclone of
high pressure
Rain-shadow effect of Rockies and
Andes

				
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