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Tropical Environments
Advanced Physical Geography Options
Cambridge International AS and A Level Geography
for Syllabus 9696
Tropical environment = area between 23.5°N and 23.5°S. Area covers 50 million KM² land, half
in Africa. 3 types tropical climates(A):
- Rainforest climates (Af)
- Monsoon climates (Am)
- Savanna climates (Aw)
Air masses
Original concept, large body air physical properties = temp. + humidity, uniform over AREA.
Now defined as large body air, horizontal gradients of main physical properties slack. Generally
applied only lower layers atmosphere. Air masses cover thousands km².
They derive temp. + humidity from regions which lie (source regions) principal ones:
- Areas relative calm, like semi-permanent high-pressure areas
- Where surface uniform, deserts, oceans + ice-fields
Air masses can modify when leave source.
First classification Air masses made by Bergeron 1928. Classified first latitude of source area
(controls temp.) next whether source area continental(dry) or maritime(moist). Maritime
tropical(mT) one that warm + moist. Next is subdivision refers stability of mass, whether cooled
+ become stabler, or become warmer + less stable.
As mass move from source, may change cause of:
- Internal changes
- Effects of surface which move over
Changes create secondary air masses. e.g. warm mass travels over cold surface, cools + more
stable. Low cloud or fog low chance rain. Cold air mass passes over warm surface warmed + less
stable. Rising air likely produce rain. Air masses that warmed = ‘W’. Air that cooled = ‘K’.
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Intertropical convergence zone (ITCZ)
Winds between tropics converge on ITCZ line (band few hundred km wide, enclosing places
where wind flows inwards + subsequently rise convectively)
ITCZ lies 5°N average. Known as ‘meteorological equator’. Wanders seasonally, lags two months
behind change in overhead sun. Latitudinal variation pronounced over Indian Ocean cause large
Asian continental land mass to N. Over E. Atlantic + E. Pacific Oceans, ITCZ moves seasonally
due to cold Benguela + Peru ocean currents.
Movement of ITCZ over S. Africa complicated by land's shape, elevation + location. Southerly
spur of ITCZ known as ‘Zaire Air Boundary’ (ZAB). Largest + prominent spur in S. Pacific = S.
Pacific Convergence Zone (SPCZ) is warm pool near Papua. + most pronounced in Summer. Lies
mainly over water. Is convergence of:
- Moist northerlies from semi-permanent high pressure in S.E. Pacific
- S.E. easterlies from mobile highs moving across S.W. Pacific in summer.
Winds at ITCZ light or non-existent, creating calm conditions called doldrums. Thought,
occasionally bursts strong Westerlies, ‘westerly wind bursts.
Subtropical anticyclones
Centres or ridges high pressure = subsidising air + cold atmosphere. Found over continents,
mainly in winter. High pressure belt S.E. Australia in winter, high pressure S. Australia in
summer when sea colder.
Subtropical high/warm anticyclone caused by cold air descending at tropopause. 2 rings high
pressure = 30-35° North + South. Position high pressure cause of subsiding of Hadley cell. Alters
5-10° with movement of ITCZ.
Subtropical high-pressure belts lie over ocean, especially summer. Subtropical high anchored
over E. Pacific by anticyclone from Andes. High strong cause of cold ocean surface. El Niño
Events doesn't occur. Highs larger than low pressure systems, 4,000 km width 2,000 km N/S.
Smaller pressure gradients involved, winds lighter. Subtropical high-pressure belt crossed with
cold fronts. Move E. at speed 30-50 km/h
Anticyclone movement may stall + travel less 20° latitude in week, known as ‘blocking
anticyclone’
Effects
Highs at surface cause subsidence. Temp. inversion occur, where cold high in winter over
continent. Where low air pollution + moisture, low level stratus clouds form, cause anticyclone
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gloom. E.g. Melbourne in winter. Arid climates = prevalence high pressure like N.E. Brazil.
Protrudes enough into S. Atlantic to be dominated by high pressure.
Ocean Currents
Oceanic gyre explains why E. Coast in S. hemisphere + W. coast in N. hemisphere usually warm
+ wet, warm currents carry water pole wards + raise air temp. maritime areas. Cold currents
carry water towards equator, lower temp. coastal areas. West Coast cool + dry as:
- Advection cold water from poles
- Cold upwelling currents.
E.g. S. Africa E. coast 3-8°c warmer than W. coast.
Wind
Temp. wind determined by origin of air + characteristics surface which blows. Wind from sea
warmer in winter, cooler in summer than wind from land.
Monsoon
Wind patterns that experience seasonal reversal. Occur India, E. África, Arabia, Australia, China.
Basic cause = difference in heating land + sea on continental scale.
In India 2 main seasons observed:
- N.E. monsoon, winter season + hot dry season.
- S.W. monsoon, rainy season + post monsoon season.
Most India's rainfall during S.W. monsoon.
Winter season, winds blow outwards as high pressure centred over land. Parts S. India + Sri
Lanka receive rain, N.W. India receive rain as depression. Winter rains important as allow grow
cereals. Mean temp. in winter = 26°c Sri Lanka, 10°c Punjab. N. regions + interior areas larger
temp. range than coastal. In N. daytime temp. over 26°c while frost at night. Hot, dry season
March-May. Spreads N. through India. Daytime in N. = +49°c while coastal hot + humid. Veg.
growth prevented + rivers dry up. In spring, high pressure over India replaced low pressure. Low
pressure over equator, little regional air circulation. Man storms + dust. Increase humidity near
coast = rain. Sri Lanka, S. India, Bay Bengal receive rain = rice + tea. Most India has continued
drought. Rainy season = low pressure system intensifies. When pressure low enough, air from
equatorial low + S. hemisphere suckled in, bringing Moist air. As passes over ocean more
moisture + heavy rain when over India.
S.W. monsoon in S. India early June, by end month Country affected, peaks in July-August.
Rainfall varied between windward + leeward sites. Low pressure over N.W. India driest part,
