Five climate types with letter codes — the world standard
A (Tropical): always hot and wet. B (Dry): deserts and steppes. C (Temperate): mild winters. D (Continental): cold winters, warm summers. E (Polar): always cold, warmest month below 10°C.
A
Tropical — hot and wet year-round
B
Dry — desert and steppe
C
Temperate — mild winters
D
Continental — cold winters
E
Polar — always cold
Atmospheric Circulation
ITCZ (Inter-Tropical Convergence Zone): equator gets most sun → air rises → rain. Subtropics: air descends → deserts.
Atmospheric Circulation
Why the equator is rainy and 30° latitude is desert
Intense solar heating at the equator forces air to rise, cool, and dump rain → tropical forests. At ~30°, air descends dry → deserts (Sahara, Arabian, Atacama, Sonoran).
El Niño / La Niña
El Niño: warm eastern Pacific → drought in Australia, flooding in Peru
El Niño / La Niña
Periodic Pacific Ocean warming and cooling disrupts global weather
El Niño (warm): weakens trade winds, warm water shifts east → Australian drought, Peruvian flooding. La Niña (cool): opposite effects. Occurs every 2–7 years. Affects global agriculture.
Four major greenhouse gases and how they warm the planet
Water vapor most abundant. CO₂ most significant human-emitted. Methane 80× more potent than CO₂ over 20 years. Nitrous oxide from fertilizers and livestock.
CO₂
Carbon dioxide — fossil fuels
CH₄
Methane — livestock, landfills
N₂O
Nitrous oxide — agriculture
H₂O
Water vapor — most abundant
Urban Heat Island Effect
Urban heat island: cities are warmer than surrounding areas — concrete and asphalt absorb heat
Urban Heat Island Effect
Why cities are consistently warmer than nearby rural areas
Dark paved surfaces absorb more solar radiation than vegetation. Buildings trap heat and block wind. Lack of vegetation reduces evaporative cooling. Cities can be 1–3°C warmer than surrounding areas.
Atmospheric Circulation Cells
Hadley cell: equatorial air rises → moves poleward → descends at ~30° → returns to equator as trade winds
Atmospheric Circulation Cells
The three circulation cells that drive global wind patterns
Hadley cell (0-30°): equatorial heating drives rising air, poleward flow at altitude, subsidence at ~30° (deserts), return as trade winds. Ferrel cell (30-60°): mid-latitude circulation, westerly winds. Polar cell (60-90°): cold polar air descends, flows equatorward. Boundaries create major climate zones.
How ocean circulation moderates and shapes regional climates
Warm currents flow poleward on western sides of ocean basins (Gulf Stream, Kuroshio). Moderate adjacent coastlines — NW Europe warmer than its latitude suggests. Cold currents flow equatorward on eastern sides (California, Peru/Humboldt, Benguela). Upwelling brings cold, nutrient-rich water → coastal deserts, rich fisheries.
Monsoon Systems
Monsoon: seasonal reversal of winds → wet summer, dry winter. South Asia most affected.
Monsoon Systems
Seasonal wind reversals that dominate South and East Asian climates
Summer monsoon: land heats faster than ocean → low pressure over land → moist ocean air drawn inland → heavy rain. Winter monsoon: land cools faster → high pressure over land → dry continental air flows out to sea. South Asian monsoon: June-September, critical for agriculture. 1.5 billion people depend on monsoon rains.
Sea level rise: thermal expansion + ice melt. ~3.3mm/year currently, accelerating. Threatens coastal cities, small island nations. Extreme weather: more intense hurricanes, longer droughts, heavier precipitation events. Biome shifts: species ranges moving poleward and upslope. Ocean acidification: CO₂ absorption → carbonic acid → threatens coral reefs and shellfish.
Sea level
Thermal expansion + ice melt — threatens coasts
Extreme weather
More intense storms and droughts
Biome shifts
Species moving poleward
Acidification
CO₂ → carbonic acid → coral bleaching
Microclimate
Microclimate: local climate conditions that differ from surrounding area. Urban heat island is one example.
Microclimate
How local conditions create distinct climate zones within larger regions
Microclimates created by: topography (south-facing slopes warmer in N. hemisphere), water bodies (lake effect snow, coastal fog), vegetation (forest canopy modifies temperature and humidity), urban surfaces (heat island). Frost hollows: cold air sinks into valleys at night. Understanding microclimates is crucial for agriculture and urban planning.
Coriolis Effect
Coriolis effect: moving objects deflect right in Northern Hemisphere, left in Southern Hemisphere
Coriolis Effect
Why winds and ocean currents curve — and why it matters
Earth's rotation deflects moving objects (winds, currents, projectiles) to the right in the Northern Hemisphere and left in the Southern. NOT strong enough to affect water draining from bathtubs — that's a myth. Does affect: hurricanes/cyclones (counterclockwise in NH, clockwise in SH), trade winds, jet streams.
Acid Rain
Acid rain: SO2 (sulfur dioxide) and NOx (nitrogen oxides) from burning fossil fuels combine with water vapor → sulfuric and nitric acid
Acid Rain
How air pollution causes chemical weathering far from its source
Power plants and vehicles emit SO₂ and NOₓ. These react with water vapor and oxygen to form sulfuric and nitric acids. Fall as precipitation far downwind. Effects: acidifies lakes (kills fish), damages forests, erodes limestone buildings and sculptures. Transboundary pollution — Canada affected by US emissions, Scandinavia by UK/Germany.
Mnemonic
What it means
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🎓 Common Exam Questions
Q: What does the Koppen ABCDE climate system classify?
A: Koppen (A=Tropical, B=Dry, C=Temperate, D=Continental, E=Polar): A Tropical — no cold season, all months above 18 degrees C, high rainfall. Subtypes: Af (rainforest), Aw (savanna). B Dry — evaporation exceeds precipitation. Subtypes: BWh (hot desert — Sahara), BSh (hot steppe). C Temperate — mild winters, warm summers. Includes Mediterranean (Cs), Humid Subtropical (Cfa), Oceanic (Cfb). D Continental — cold winters below 0 degrees C, large seasonal swings. E Polar — no month above 10 degrees C. ET (tundra), EF (ice cap). Classification determines vegetation, agriculture, and settlement patterns.
Q: Explain the ITCZ (Inter-Tropical Convergence Zone) and global precipitation patterns.
A: ITCZ (Inter-Tropical Convergence Zone): low pressure band near equator where northeast and southeast trade winds meet. Intense solar heating causes air to rise rapidly, cool, and produce heavy convective rainfall — tropical rainforests thrive here. ITCZ migrates seasonally following the sun — northward in Northern Hemisphere summer. This creates wet and dry seasons in tropical regions. At 30 degrees latitude, descending air creates subtropical high pressure — desert belts (Sahara, Arabian Desert, Australian Outback). Understanding ITCZ explains why rainforests are at the equator and deserts at 30 degrees.
Q: Explain the Hadley, Ferrel, and Polar circulation cells.
A: Three cells per hemisphere: Hadley cell (0-30 degrees): equatorial air rises (ITCZ), moves poleward, descends at 30 degrees (subtropical deserts), returns as trade winds. Ferrel cell (30-60 degrees): mid-latitude cell, surface westerlies flow poleward, creates temperate climates with variable weather. Polar cell (60-90 degrees): cold dense air descends at poles, moves equatorward as polar easterlies, meets westerlies at polar front. These three cells explain the major climate zones, vegetation belts, and desert locations — atmospheric circulation is the master control of climate geography.
Q: What is the Coriolis effect and how does it shape weather systems?
A: Earth rotates west to east. Objects moving across the surface appear to deflect — right in the Northern Hemisphere, left in the Southern. This is not a real force but an artifact of a rotating reference frame. Effects: winds curve rather than blowing straight from high to low pressure. Northern Hemisphere: clockwise rotation around high pressure (anticyclones), counterclockwise around low pressure and hurricanes. Southern Hemisphere: opposite. Trade winds deflect westward. Drives major ocean gyres. Stronger at poles, zero at equator. Hurricanes rotate counterclockwise in NH, clockwise in SH.
Q: What is the difference between weather and climate, and what is a microclimate?
A: Weather: short-term atmospheric conditions at a specific place and time. Climate: long-term average of weather over 30 plus years. The saying: climate is what you expect, weather is what you get. Climate change refers to shifts in long-term averages. Microclimate: local conditions differing from surrounding area due to local factors. Examples: south-facing slopes are warmer than north-facing; cities are warmer than surrounding farmland (urban heat island); valleys prone to frost when surrounding hills are not (cold air drainage); coastal areas more moderate than inland. Microclimates are crucial for agriculture (viticulture), urban planning, and ecological management.