EVERGREEN SELVAS
PARKLAND TALL GRASS
SUBSIDENCE & XEROPHYTES
Global Climate I
Equatorial, Monsoon & Savanna (Group A) · Hot Subtropical & Mid-Latitude Cold Deserts (Group B) (28 Micro-Realms)
Tropical Humidity & Arid Extremes (Köppen Groups A & B)
The global distribution of climate begins at the Equator and spans outward. Köppen's Group A (Tropical) climates never experience a month below 18°C. This includes the uniformly wet and hot Equatorial (Af) climate sustaining dense Amazonian Selvas; the Monsoon (Am) characterized by seasonal wind reversals and deciduous forests; and the Savanna/Sudan (Aw) type featuring distinct wet/dry seasons and "parkland" tall grass (Big Game Country). Köppen's Group B (Dry) climates exist where evaporation exceeds precipitation. These include the Hot Subtropical Deserts (BWh) locked on the western coasts between 15°–30° due to Hadley subsidence and cold ocean currents (Sahara, Atacama), and the Cold Mid-Latitude Deserts (BWk) isolated deep in continental interiors or extreme rainshadows (Gobi, Patagonia).
I. Köppen Group A & Equatorial Climate (Af)
Group A: Tropical Humid
- Exist between the Tropic of Cancer & Capricorn
- Average temp of the coldest month is >18°C
- Abundant sunshine and high annual temperature
The Three A Subtypes
- Af: Wet all year round (Equatorial)
- Am: Short dry season, heavy monsoon rain
- Aw: Distinct, pronounced winter dry season
Equatorial Climate (Af)
- Located between $5^\circ\text{–}10^\circ$ North and South
- Uniformly hot and wet throughout the year
- No distinct seasons; diurnal range > annual range
The "4 O'Clock Showers"
- Mornings are bright and sunny
- Intense convection causes towering Cumulonimbus
- Heavy torrential downpour with lightning around 4 PM daily
Zero Annual Range
- Temperature curve is practically a flat horizontal line
- Mean monthly temperatures hover around $27^\circ\text{C}$ constantly
- "Winter is unknown" in the equatorial zone
Selvas (Evergreen Forests)
- High heat & moisture support luxuriant rainforests (Selvas)
- Layered canopy blocks sunlight from reaching the floor
- Hardwoods (Mahogany, Ebony), Lianas, Epiphytes
Economic Development
- Commercial lumbering is difficult (trees don't grow in pure stands)
- Oppressive climate breeds diseases (Malaria, Tsetse fly)
- Tribes (Pygmies, Amazonians) practice shifting cultivation
II. Tropical Monsoon (Am) & Savanna/Sudan (Aw)
Tropical Monsoon (Am)
- Complete reversal of wind direction between seasons
- Summer: Onshore SW winds bring heavy rain
- Winter: Offshore NE winds bring dry conditions
Three Distinct Seasons
- Unlike the uniform Equatorial climate
- Monsoon lands experience three pronounced seasons
- Rhythm dictated by the migration of the ITCZ
Monsoon Distribution
- Indian Subcontinent is the core region
- South-East Asia (Myanmar, Thailand, Vietnam)
- Northern Australia & parts of West Africa
Monsoon Deciduous Forest
- Trees shed their leaves during the hot, dry season
- To conserve water through transpiration
- Teak, Sal, Sandalwood, Bamboo (Commercially viable!)
Tropical Savanna/Sudan (Aw)
- Located between $10^\circ$ and $20^\circ$ N/S latitudes
- Transitional climate between Equatorial and Deserts
- Best developed in the Sudan region of Africa
The Parkland Landscape
- Characterized by tall, coarse grass (Elephant grass)
- Interspersed with short, scattered, flat-topped trees (Acacia, Baobab)
- Continuous forest canopy cannot survive the long dry winter
"Big Game Country"
- Vast grasslands support millions of herbivores (Zebra, Wildebeest)
- Hunted by apex carnivores (Lions, Cheetahs)
- Hausa (cultivators) & Masai (pastoralists) tribes
III. Köppen Group B & Hot Subtropical Deserts (BWh)
Group B: Dry Climates
- Not defined solely by temperature, but by ARIDITY
- Evaporation outpaces precipitation all year round
- Permanent water deficit prevents tree growth
BWh vs. BWk
- W = Wüste (German for Desert); S = Steppe (Semi-arid)
- h = hot (mean annual temp > 18°C)
- k = cold (kalt) (mean annual temp < 18°C)
Hot Subtropical Desert (BWh)
- The world's major hot deserts
- Located between $15^\circ\text{–}30^\circ$ North and South
- Characterized by extreme heat and absolute drought
West Coast Location Law
DESERT
RAIN
- Why are they specifically on the WESTERN margins of continents?
- Due to three combined geographical factors! (See below)
Factor 1: Subtropical High
- Located exactly under the descending limb of Hadley Cell
- Sinking air compresses, warms adiabatically, and dries out
- Impossible for rain-bearing clouds to rise/form
Factor 2: Offshore Trades
- Trade winds blow from East to West
- They dump all their moisture on the East coasts
- By the time they reach the West coasts, they are completely dry
Factor 3: Cold Ocean Currents
- West coasts are flanked by cold ocean currents (Canary, Humboldt, Benguela)
- Chill the air from below, causing temperature inversion
- Prevents vertical convection $\to$ fog forms, but no rain!
IV. Mid-Latitude Cold Deserts (BWk) & Arid Adaptations
Mid-Latitude Deserts (BWk)
- Located in temperate zones, far from the oceans
- Unlike BWh, they experience severe, freezing winters
- Aridity is caused by Continentality, not Hadley subsidence
The Rainshadow Effect
RAINSHADOW
- Often ringed by massive mountain barriers
- Which block all moisture-bearing marine winds
- Resulting in an absolute rainshadow basin
Gobi & Patagonia
- Gobi (Mongolia/China): Blocked by the Himalayas/Tibet
- Patagonia (Argentina): Blocked by the Andes from Westerlies
- Also Great Basin (USA), Turkestan Desert
Xerophytic Vegetation
- Plants adapted to survive extreme drought (Xerophytes)
- Long tap roots to reach deep groundwater
- Leaves reduced to spines (Cacti) to prevent transpiration
BWh: Diurnal Temp Range
(SCORCHING)
(FREEZING)
- Hot deserts have the highest DIURNAL (daily) temp range
- Clear skies allow intense daytime heating
- And rapid nighttime radiative cooling
BWk: Annual Temp Range
- Cold deserts have a massive ANNUAL temp range
- Due to extreme continentality
- Scorching summers, but brutal, snow-covered winters
Master Group A & B Matrix
- Completes Köppen Tropical & Arid climates
- Locks the West Coast Desert trap logic
- Transitions to Realm 84 (Global Climates II - Temperate/Polar)
Core Distinctions
• Equatorial (Af): No dry season. It rains heavily almost every afternoon ("4 O'clock showers"). The diurnal temperature range is greater than the annual range. "Winter is unknown." Supports Evergreen Selvas.
• Savanna (Aw): Distinct Wet Summer and Dry Winter. The prolonged dry season prevents a continuous forest canopy from surviving, creating a landscape of tall grass and scattered short trees (Parkland).
Hot deserts are found strictly on the western margins of continents between 15°–30° because of a triad of factors:
1. They sit under the subsiding (sinking), dry air of the Subtropical High-Pressure Belt.
2. The Trade Winds blow offshore, having dumped their moisture on the east coasts.
3. Flanked by Cold Ocean Currents (like the Humboldt or Benguela), which chill the air from below, creating a stable thermal inversion that prevents rain-bearing clouds from rising.
Hot deserts (Sahara) owe their aridity to atmospheric subsidence and trade winds. Cold deserts (Gobi, Patagonia) owe their aridity to extreme continentality and being located deep inside massive rain shadows of high mountain ranges. BWk deserts experience freezing winters.
Climate System Matrix
Köppen Group A & B Reference