Major Forms of Condensation

What are the forms of condensation? n Cloud n Fog n Frost and Dew

Differences between Forms of Condensation n Cloud, fog, frost and dew?? –Cloudhigh in the sky –fognear the surface of the earth –frost and dewat the surface of the earth

Similar in Formation n Cooling (Temperature drops) n Air holds less water n RH increases n 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) n Clear sky n high RH n temperature inversion n calm and still air n presence of depression or hollow

Advection Fog

Frontal Fog

Steam Fog

Upslope Fog

CLOUD

Precipitation n Water droplets from condensation is very small in size – 0.1 mm n Raindrops are much larger – mm n The main difference between condensation and precipitation is the size of water droplets

Formation Processes n Air cooling n Condensation and cloud formation n Accumulation of moisture n The growth of cloud droplets

Collision and Coalescence n In tropical and sub-tropical regions n Cloud top temperature : n warmer than -15 o C n Turbulence Mixing : n Different size droplets move at different speeds n cumulonimbus

Collision and Coalescence Large Water droplet Fall in higher speed Fall in lower speed Direct capture wake capture

Ice-Crystal (Bergeron) Process n In higher latitudes (temperate regions) n Super-cooled water – water droplets in cloud with temperature below 0 o C n Because of extremely small size of cloud droplets n Insufficient freezing nuclei

Ice-Crystal (Bergeron) Process n Saturated vapour pressure over super-cooled water is greater than that over ice

Ice-Crystal (Bergeron) Process n Air can be saturated for ice when it is not saturated for water

Ice-Crystal (Bergeron) Process n Ice crystals grow at the expense of cloud droplets until they are large enough to fall

3 main types of precipitation Convection Rain Cyclonic / Frontal rain Orographic / relief rain Lifting mechanism Real world examples Diagram of formation Low pressure system due to thermal nature Warm air mass meets cold air mass Uplifting along slopes ITCZPolar front Tai Mo Shan

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

Spatial Variation of rainfall n Primary maximum occurs at a belt between 10 o N to 10 o S in the vicinity of the ITCZ ocean continent

Spatial Variation of rainfall n secondary maximum occurs at a belt between 40 o to 55 o N&S in the vicinity of the ITCZ ocean continent

Spatial Variation of rainfall n Primary minima find in latitudes higher than 55 o N&S n Secondary minima in sub- tropical high pressure belts (20 o -35 o N&S) ocean continent

Factors affecting the pattern of rainfall n Air pressure n Zones of convergence and divergence n Strong convection along low pressure belts n Uprising air leads to condensation and cloud formation n Descending air along high pressure belts suppress condensation

n n Describe and explain the factor that influence the pattern of rainfall from the diagram on the right. 0o0o 50 o S 50 o N

Zones of abundant rainfall n Around the equatorial regions n e.g. Amazon Basin n Doldrum low pressure n Convergence of trade winds along ITCZ n Intense solar radiation promotes strong convective uplift n Ares dominated by warm moist equatorial marine air masses

Zones of abundant rainfall n Western sides in middle latitudes n e.g. Western Coast of Canada n Under influences of the onshore westerlies and frequent frontal cyclones n Western mountrains(Rockies and Andes) promote orographic influences and heavy precipitation

n n Describe and explain the factor that influence the pattern of rainfall from the diagram on the right.

Zones of scanty rainfall n Polar areas of North America n Air with low moisture content due to low temperature n Descending cold upper air masses unfavourable for condensation

Zones of scanty rainfall n 15 o -30 o S in the western side of South America n 30 o -40 o N of south-western U.S.A. n Affected by sub-tropical anticyclone of high pressure n Rain-shadow effect of Rockies and Andes