World Geography
🔒 Log in to trackAtmosphere, pressure belts, planetary and local winds
🔒 Log in to trackComposition (dry air by volume)
Nitrogen ~78%, Oxygen ~21%, Argon ~0.93%, Carbon dioxide ~0.04%, plus traces of neon, helium, ozone and variable water vapour.
Layers of the atmosphere (from the surface up)
| Layer | Key facts |
|---|---|
| Troposphere | All weather; temperature falls about 6.5 °C per km (normal lapse rate); about 8 km thick at poles, 18 km at equator; ends at the tropopause |
| Stratosphere | Contains the ozone layer (absorbs UV); calm air — preferred by jet aircraft |
| Mesosphere | Coldest layer; meteors burn here |
| Thermosphere | Contains the ionosphere, which reflects radio waves; auroras |
| Exosphere | Outermost, very thin air merging into space |
Pressure belts and planetary winds
- Equatorial low (doldrums), subtropical highs (horse latitudes, ~30°), sub-polar lows (~60°), polar highs.
- Trade winds blow from subtropical highs to the equator (NE trades in the north, SE trades in the south).
- Westerlies blow from subtropical highs towards 60°; in the Southern Hemisphere they are called the Roaring Forties, Furious Fifties and Shrieking Sixties.
- Coriolis force deflects winds to the right in the Northern and left in the Southern Hemisphere (Ferrel's law).
Local winds (very common PYQ)
| Wind | Region | Nature |
|---|---|---|
| Chinook | Rocky Mountains (USA–Canada) | Warm, dry; 'snow eater' |
| Foehn | Alps (Switzerland) | Warm, dry; helps grapes ripen |
| Sirocco | Sahara → Italy, Mediterranean | Hot, dusty; 'blood rain' |
| Harmattan | West Africa | Dry, dusty from the Sahara; called 'The Doctor' |
| Mistral | Rhône valley, France | Cold, northerly |
| Bora | Adriatic coast | Cold, dry |
| Khamsin | Egypt | Hot, dusty |
| Santa Ana | Southern California | Hot, dry; spreads wildfires |
| Brickfielder | Australia (Victoria) | Hot, dry |
| Loo | North India | Hot, dry summer wind |
Tropical cyclones — same storm, different names
Hurricane (Atlantic, Caribbean), Typhoon (north-west Pacific — China, Japan, Philippines), Cyclone (Indian Ocean), Willy-willy (older term for north-west Australia). A tornado is a small, violent land funnel, most frequent in the USA.
Detailed notes
What the atmosphere is made of
The atmosphere is the blanket of air held by gravity. Dry air by volume: Nitrogen 78%, Oxygen 21%, Argon 0.93%, Carbon dioxide about 0.04%, plus traces of neon, helium and ozone, and variable water vapour and dust. Water vapour and dust matter because clouds and rain form around them; CO2 matters because it traps heat (the greenhouse effect, together with methane and water vapour).
The five layers - bottom to top
| Layer | Height (about) | Remember for |
|---|---|---|
| Troposphere | 8 km (poles) - 18 km (equator) | All weather occurs here; temperature falls 6.5 degrees C per km (normal lapse rate); ends at the tropopause |
| Stratosphere | up to 50 km | Ozone layer (15-35 km) absorbs ultraviolet rays, so temperature rises with height; calm air - jet aircraft fly here |
| Mesosphere | up to 80-85 km | Meteors burn up here; coldest layer (about -90 degrees C) |
| Thermosphere (Ionosphere) | up to about 600 km | Electrically charged ions reflect radio waves; auroras (northern/southern lights); the ISS orbits at about 400 km |
| Exosphere | beyond | Outermost; air so thin that satellites pass through almost freely |
Ozone (O3) shields life from UV-B rays. Its thinning - the 'ozone hole' over Antarctica (first reported in 1985) - was caused mainly by CFCs; the Montreal Protocol (1987) phases them out, and the layer is slowly healing.
Pressure belts and planetary winds
Air pressure falls with height but also varies with temperature. Four classic belts circle the Earth:
- Equatorial low (Doldrums, 0-5 degrees) - rising warm air, daily convectional rain, calm winds.
- Subtropical high (Horse latitudes, 25-35 degrees) - sinking air, calm, dry - the great deserts (Sahara, Thar, Australian) lie under it.
- Subpolar low (about 60 degrees) - stormy convergence zone.
- Polar high - cold, dense, sinking air. The equatorial low and polar high have a thermal origin (temperature); the other two are dynamic (formed by descending/converging air).
Winds blow from high to low pressure and are bent by the Earth's rotation (right in the Northern Hemisphere, left in the Southern - the Coriolis effect). Three planetary belts: Trade winds (from subtropical highs to the equatorial low - north-east trades in the north, south-east trades in the south; steady and reliable), Westerlies (35-60 degrees; in the southern ocean they are the Roaring Forties, Furious Fifties and Shrieking Sixties, because there is little land to break them), and Polar easterlies. The trade winds of the two hemispheres meet along the ITCZ (Inter-Tropical Convergence Zone), which follows the Sun and gives the monsoon its seasonal reversal.
Cyclones, local winds and rainfall types
Tropical cyclones are violent spirals born over warm seas between 5-20 degrees latitude. Names by region: Hurricane (Atlantic, Caribbean, east Pacific), Typhoon (China Sea, north-west Pacific), Willy-Willy (Australia), simply Cyclone (Indian Ocean). A tornado is a much smaller funnel vortex of the American midwest ('twister') - not a tropical cyclone. Temperate (frontal) cyclones form along fronts in mid-latitudes; the winter western disturbances that rain on north India are weak temperate systems.
Local winds worth a flashcard set - Hot: Loo (India-Pakistan), Sirocco (Sahara to Italy, carrying red dust - 'blood rain'), Khamsin (Egypt), Brickfielder (Australia), Zonda (Argentina), Chinook ('snow eater' on the east slope of the Rockies) and Foehn (Alps - melts snow, ripens grapes). Cold: Mistral (Rhone valley, France), Bora (Adriatic), Blizzard (snow-laden, polar), Buran (Russia), Pampero (pampas of Argentina), Harmattan (dry, dusty West African wind, called 'The Doctor'). Rain falls in three ways - convectional (daily afternoon showers of the equator), orographic/relief (windward slopes - Cherrapunji's rain) and cyclonic/frontal.
Quick revision
- Air: N2 78, O2 21, Ar 0.93, CO2 0.04 (%); weather = troposphere; ozone = stratosphere; meteors burn = mesosphere; aurora/radio = ionosphere.
- Lapse rate 6.5 degrees C per km; jets fly in the stratosphere; coldest layer = mesosphere.
- Belts: Doldrums (equatorial low), Horse latitudes (subtropical high); trades, westerlies, polar easterlies.
- Roaring Forties etc. = southern westerlies; ITCZ moves with the Sun - monsoon link.
- Hurricane (Atlantic), Typhoon (NW Pacific), Willy-Willy (Australia); tornado is different.
- Chinook snow-eater, Foehn Alps, Mistral France, Harmattan West Africa 'Doctor'.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Composition and layers of the atmospherevery common3 practice Q
'Which gas is most abundant', 'weather occurs in which layer', lapse-rate and layer-feature questions, aurora/ozone/meteor placement.
- Dry air: N2 78%, O2 21%, Ar 0.93%, CO2 0.04%; water vapour and dust vary.
- Layers bottom-up: Troposphere (all weather; lapse rate 6.5 C/km) - Stratosphere (ozone 15-35 km; jets) - Mesosphere (meteors burn; coldest) - Thermosphere/Ionosphere (aurora, radio reflection, ISS) - Exosphere.
- Place each fact in its layer before looking at options.
Example: The normal lapse rate - the fall of temperature with height in the troposphere - is about:
6.5 degrees C per km. All weather is confined to the troposphere, whose top (tropopause) lies at 8 km over the poles and 18 km over the equator.
Type 2: Ozone layer and environmental factscommon3 practice Q
'Ozone hole appears over', 'Montreal Protocol', 'which gas absorbs ultraviolet rays', CFC questions.
- Ozone (O3) in the stratosphere (15-35 km) absorbs harmful UV rays; it is a trace gas, not a layer of pure gas.
- The 'ozone hole' appears seasonally over Antarctica (first reported 1985); the main culprits are CFCs.
- The Montreal Protocol (1987) phased out CFCs, and the layer is slowly recovering.
Example: The ozone hole was first detected over which region?
Antarctica - a springtime thinning of the ozone layer, blamed mainly on man-made CFCs and checked by the Montreal Protocol of 1987.
Type 3: Pressure belts and planetary windsvery common3 practice Q
'Doldrums lie at', 'Roaring Forties are', 'trade winds blow from', horse latitudes, ITCZ, wind deflection.
- Belts: equatorial low (Doldrums), subtropical high (Horse latitudes), subpolar low, polar high; deserts sit under the subtropical highs.
- Winds: trades (NE in the north, SE in the south, towards the equator), westerlies (35-60 degrees; southern ones = Roaring Forties, Furious Fifties, Shrieking Sixties), polar easterlies.
- Rotation deflects winds right in the Northern and left in the Southern Hemisphere; the ITCZ (where trades meet) follows the Sun - the basis of the monsoon.
Example: The 'Roaring Forties' are strong winds found in the:
Southern Hemisphere westerlies (40-50 degrees S) - so named because almost unbroken ocean lets them blow at full strength.
Type 4: Cyclones: names by region and typesvery common3 practice Q
'Typhoons occur in', 'willy-willy is a cyclone of', hurricane/typhoon/cyclone matching, tornado identification, temperate cyclones.
- Tropical cyclone names: Hurricane - Atlantic/Caribbean and east Pacific; Typhoon - China Sea/north-west Pacific; Willy-Willy - Australia; Cyclone - Indian Ocean.
- A tornado is a small, violent funnel (USA midwest, 'twister') - not a tropical cyclone.
- Temperate (frontal) cyclones form in mid-latitudes; India's winter 'western disturbances' are weak temperate systems that rain on Punjab-Haryana.
Example: Tropical cyclones of the China Sea and the north-west Pacific are called:
Typhoons. The same storms are hurricanes in the Atlantic and willy-willies off Australia.
Type 5: Local winds: name ↔ region ↔ charactervery common3 practice Q
'Chinook is a wind of', 'Harmattan is called the Doctor', 'which wind melts snow', hot/cold local wind matching.
- Warm: Loo (N. India summer), Sirocco (Sahara to Italy, red dust), Khamsin (Egypt), Chinook ('snow eater', east of the Rockies), Foehn (Alps - melts snow, ripens vines), Brickfielder (Australia), Zonda (Argentina).
- Cold: Mistral (Rhone valley, France), Bora (Adriatic), Blizzard (snow-laden), Buran (Russia), Pampero (pampas), Harmattan (dry, dusty West Africa - 'The Doctor').
- Match region first, then hot/cold character; distractors usually come from the other hemisphere.
Example: The warm, dry Foehn wind that melts snow on the lee side of mountains blows over which region?
The Alps (Europe). Its North American equivalent on the east slope of the Rockies is the Chinook, the 'snow eater'.
Shortcut tricks
⚡ Layer order: 'The Strong Man Throws Eggs'
Troposphere → Stratosphere (ozone, jets) → Mesosphere (coldest, meteors) → Thermosphere (ionosphere, radio) → Exosphere.
Example: In which layer do meteors burn up?
The mesosphere.
⚡ Local wind by country
Chinook–USA/Canada, Foehn–Alps, Sirocco–Sahara→Italy, Harmattan–West Africa, Mistral–France, Khamsin–Egypt, Santa Ana–California, Loo–India.
Where students lose marks
Placing the ozone layer in the troposphere — it is in the stratosphere.
Calling the Harmattan a cold wind — it is dry and dusty; 'Doctor' because it relieves humid heat.
Saying Coriolis deflects to the left in the Northern Hemisphere — it is to the right.
Practice sets — 18 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.