World Geography
🔒 Log in to trackAtmosphere, pressure belts, planetary and local winds
🔒 Log in to trackThe atmosphere is a blanket of air held by gravity. It has five layers, and all weather lies in the lowest one. Pressure belts drive the planetary winds. Every region also has famous local winds and its own name for a cyclone.
What the air is made of
Dry air by volume:
| Gas | Share |
|---|---|
| Nitrogen | about 78% |
| Oxygen | about 21% |
| Argon | about 0.93% |
| Carbon dioxide | about 0.04% |
Neon, helium and ozone are traces. Water vapour and dust vary. Clouds and rain form around them. CO2 traps heat. That is the greenhouse effect, together with methane and water vapour.
Five layers from the ground up
| Layer | Height (about) | Remember for |
|---|---|---|
| Troposphere | 8 km (poles) to 18 km (equator) | All weather. 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 UV. Calm air, so jets fly here |
| Mesosphere | Up to 80-85 km | Coldest layer (about -90 degrees C). Meteors burn up |
| Thermosphere | Up to about 600 km | Holds the ionosphere, which reflects radio waves. Auroras. The ISS orbits near 400 km |
| Exosphere | Beyond | Outermost. Air is so thin that satellites pass almost freely |
Tip: The Strong Man Throws Eggs: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.
Ozone (O3) shields life from UV-B rays. The "ozone hole" over Antarctica was first reported in 1985. CFCs caused most of it. The Montreal Protocol (1987) phases them out. The layer is slowly healing.
Pressure belts
Four belts circle the Earth.
- Equatorial low (Doldrums, 0-5 degrees): warm air rises. Daily rain, calm winds.
- Subtropical high (Horse latitudes, 25-35 degrees): air sinks. Calm and dry. The Sahara, Thar and Australian deserts lie under it.
- Subpolar low (about 60 degrees): stormy meeting zone.
- Polar high: cold, dense, sinking air.
The equatorial low and polar high are thermal, made by temperature. The other two are dynamic, made by sinking or meeting air.
Planetary winds
Winds blow from high to low pressure. Earth's spin bends them right in the Northern and left in the Southern Hemisphere. This is the Coriolis effect.
- Trade winds: from the subtropical highs to the equatorial low. North-east in the north, south-east in the south. They are steady.
- Westerlies: between 35 and 60 degrees. Over the southern ocean, few lands stop them. They are the Roaring Forties, Furious Fifties and Shrieking Sixties.
- Polar easterlies: from the polar highs.
The trade winds of both hemispheres meet at the ITCZ. It follows the Sun and gives the monsoon its seasonal turn.
Watch: The Coriolis effect bends winds to the right in the north. It is not the left.
Tropical cyclones and other storms
Tropical cyclones are violent spirals born over warm seas at 5-20 degrees latitude. The name depends on the region.
| Name | Region |
|---|---|
| Hurricane | Atlantic, Caribbean, east Pacific |
| Typhoon | China Sea, north-west Pacific |
| Willy-Willy | Australia |
| Cyclone | Indian Ocean |
A tornado is a much smaller funnel, seen in the American midwest. It is 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
| Wind | Place | Nature |
|---|---|---|
| Loo | India and Pakistan | Hot, dry summer wind |
| Sirocco | Sahara to Italy | Hot, red dust ("blood rain") |
| Khamsin | Egypt | Hot, dusty |
| Chinook | East of the Rockies | Warm. The "snow eater" |
| Foehn | Alps | Warm, dry. Melts snow, ripens grapes |
| Brickfielder | Australia | Hot, dry |
| Zonda | Argentina | Hot |
| Santa Ana | Southern California | Hot, dry, spreads wildfires |
| Mistral | Rhone valley, France | Cold |
| Bora | Adriatic | Cold, dry |
| Buran | Russia | Cold |
| Pampero | Pampas of Argentina | Cold |
| Harmattan | West Africa | Dry, dusty. Called "The Doctor" |
Types of rainfall
- Convectional: daily afternoon showers of the equator.
- Orographic (relief): rain on windward slopes. Cherrapunji's rain is of this kind.
- Cyclonic (frontal): rain along a cyclone or front.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Composition and layers of the atmosphere
'Which gas is most abundant', 'weather occurs in which layer', lapse rate, aurora, ozone, meteor placement.
Recall the gases: N2 78, O2 21, Ar 0.93, CO2 0.04.
List the layers bottom up.
Place each fact in its layer before reading the options.
Remember the lapse rate: 6.5 degrees C per km.
Each feature belongs to exactly one layer.
The normal lapse rate in the troposphere is about:
Show solutionHide solution
Temperature falls with height in the troposphere.
The fall is 6.5 degrees C per km.
6.5 degrees C per km
Ozone layer and environmental facts
'Ozone hole appears over', 'Montreal Protocol', 'which gas absorbs ultraviolet rays', CFC questions.
Ozone (O3) sits in the stratosphere, 15-35 km up.
It absorbs harmful UV rays.
The hole over Antarctica was first reported in 1985. CFCs caused it.
The Montreal Protocol (1987) phases CFCs out.
One region, one cause and one treaty answer the whole set.
The ozone hole was first reported over which region?
Show solutionHide solution
It was reported in 1985.
It appears over the southern polar land.
Antarctica
Pressure belts and planetary winds
'Doldrums lie at', 'Roaring Forties are', 'trade winds blow from', horse latitudes, ITCZ, wind deflection.
Name the belts: equatorial low, subtropical high, subpolar low, polar high.
Trades run from the subtropical high to the equator. Westerlies run from 35 to 60 degrees.
Deflection is right in the north, left in the south.
The ITCZ follows the Sun and links to the monsoon.
The belts decide where each wind blows.
The 'Roaring Forties' are strong winds found in the:
Show solutionHide solution
They are westerlies at about 40 degrees.
Ocean is almost unbroken in the south.
Southern Hemisphere westerlies
Cyclones: names by region and types
'Typhoons occur in', 'willy-willy is a cyclone of', hurricane / typhoon / cyclone matching, tornado, temperate cyclones.
Match the region: Atlantic is hurricane, China Sea is typhoon, Australia is willy-willy, Indian Ocean is cyclone.
A tornado is a small funnel, not a tropical cyclone.
Temperate cyclones form in mid-latitudes. Western disturbances are weak ones.
It is one storm with four names, so region decides the name.
Tropical cyclones of the China Sea and north-west Pacific are called:
Show solutionHide solution
The region is the China Sea and north-west Pacific.
Storms of that region are called typhoons.
Typhoons
Local winds: name, region, character
'Chinook is a wind of', 'Harmattan is called the Doctor', 'which wind melts snow', hot or cold wind matching.
Match the region first.
Then decide hot or cold: Loo, Sirocco, Chinook, Foehn are hot. Mistral, Bora, Harmattan are cold or dry.
Rule out options from the wrong hemisphere.
Region first, character second, removes most options.
The warm, dry Foehn that melts snow blows over which region?
Show solutionHide solution
Foehn is a warm mountain wind.
It is the Alps wind. Its Rockies twin is the Chinook.
The Alps
Types of rainfall
'Convectional rainfall occurs in', 'rain on the windward slope', Cherrapunji, equatorial afternoon showers.
Daily afternoon showers of the equator are convectional.
Rain on windward slopes is orographic.
Rain along a cyclone or front is cyclonic.
Each type has one setting that gives it away.
Which type of rainfall is seen on the windward slopes, as at Cherrapunji?
Show solutionHide solution
Rain on windward slopes is caused by relief.
Relief rain is orographic.
Orographic (relief) rainfall
Shortcuts that save time
Troposphere (weather), Stratosphere (ozone, jets), Mesosphere (coldest, meteors), Thermosphere (ionosphere, radio), Exosphere.
In which layer do meteors burn up?
Show solutionHide solution
Meteors burn in the coldest layer.
The coldest layer is the mesosphere.
The mesosphere
Chinook: USA and Canada. Foehn: Alps. Sirocco: Sahara to Italy. Harmattan: West Africa. Mistral: France. Khamsin: Egypt. Santa Ana: California. Loo: India.
Mistakes to avoid
Where most students lose marks on this subtopic.
Placing the ozone layer in the troposphere.
It is in the stratosphere.
Calling the Harmattan a cold wind.
It is dry and dusty. It is called 'The Doctor'.
Saying the Coriolis effect bends winds left in the north.
It bends them right in the Northern Hemisphere.
Treating a tornado as a tropical cyclone.
A tornado is a small land funnel, mostly in the USA.
Putting weather in the stratosphere.
All weather is in the troposphere.
Quick revision
Read this the night before the exam.
Air: N2 78, O2 21, Ar 0.93, CO2 0.04 percent.
Weather: troposphere. Ozone: stratosphere. Meteors burn: mesosphere. Aurora and radio waves: ionosphere.
Lapse rate is 6.5 degrees C per km. Jets fly in the stratosphere. Mesosphere is coldest.
Doldrums are the equatorial low. Horse latitudes are the subtropical high.
Roaring Forties and its kin are the southern westerlies. The ITCZ follows the Sun.
Hurricane: Atlantic. Typhoon: NW Pacific. Willy-Willy: Australia. Cyclone: Indian Ocean.
Chinook is the snow eater. Foehn is Alps. Mistral is France. Harmattan is West Africa.
Practice: 18 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.