Atmospheric Circulation and Weather Systems, Class 11 Geography

What Atmospheric Circulation and Weather Systems is about

Weightage: This chapter is part of Unit IV, Climate, of Fundamentals of Physical Geography, which carries 8 of the 70 theory marks in the school annual exam together with Composition and Structure of Atmosphere, Solar Radiation, Heat Balance and Temperature and Water in the Atmosphere. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Atmospheric pressure and its vertical variation

NCERT section Atmospheric Pressure; Vertical Variation of Pressure

Definition: the weight of a column of air over a unit area from mean sea level to the top of the atmosphere.

• Unit: millibar (mb). Average at sea level: 1,013.2 mb.
• Instruments: mercury barometer, aneroid barometer.
• Because of gravity, surface air is denser, so pressure is higher at the surface; it decreases with height (that is why we feel breathless at heights where air is rarefied).
• Variation in pressure from place to place is the primary cause of wind.

Vertical variation: in the lower atmosphere pressure falls rapidly, about 1 mb for each 10 m of height, but not always at the same rate.

Standard atmosphere (Table 9.1), from the ground up:

• Sea level: 15.2°C at 1,013.25 mb
• At 1 km: 8.7°C at 898.76 mb
• At 5 km: -17.3°C at 540.48 mb
• At 10 km: -49.7°C at 265.00 mb

Why no strong upward winds? The vertical pressure gradient force is much larger than the horizontal one, but it is balanced by a nearly equal and opposite gravitational force.

Worked example

A climber starts at a base camp where pressure is 860 mb and climbs 600 m. Using the chapter's approximate rate, estimate the pressure at the top. (1 mark)

Try it first, then show the solutionSolution

1. Pressure falls about 1 mb for every 10 m rise, so 600 m reduces it by about 60 mb.
2. 860 - 60 = 800 mb.

Answer: About 800 mb.

Horizontal distribution and world pressure belts

NCERT section Horizontal Distribution of Pressure; World Distribution of Sea Level Pressure

Isobars are lines connecting places of equal pressure, drawn at constant levels. To remove the effect of altitude, pressure at each station is reduced to sea level before comparing. Small pressure differences matter greatly for wind direction and velocity.

Pressure systems (Figure 9.1): a low is enclosed by isobars with the lowest pressure at the centre; a high with the highest at the centre.

World sea-level pressure belts (Figures 9.2, 9.3):

Belt	Latitude
Equatorial low	Near the equator
Subtropical highs	About 30°N and 30°S
Sub-polar lows	About 60°N and 60°S
Polar highs	Near the poles

The belts are not permanent: they oscillate with the apparent movement of the sun: in the northern hemisphere they move south in winter and north in summer.

Worked example

Station A at sea level reads 1,008 mb and station B at 500 m reads 958 mb. A student concludes that wind must blow from A to B. Is the student right? (2 marks)

Try it first, then show the solutionSolution

1. Pressure falls about 1 mb per 10 m, so B's reading reduced to sea level is about 958 + 50 = 1,008 mb.
2. Both stations then have about the same sea-level pressure, so the difference was only due to altitude and there is no horizontal gradient to drive wind from A to B; this is why station pressures are reduced to sea level before weather maps are drawn.

Answer: No: reduced to sea level both read about 1,008 mb, so the difference was caused by altitude alone.

Forces affecting the velocity and direction of wind

NCERT section Forces Affecting the Velocity and Direction of Wind

Horizontal surface winds respond to three forces; gravity also acts downward.

1. Pressure gradient force

• Pressure differences produce a force; the pressure gradient is the rate of change of pressure with distance.
• Strong where isobars are close; weak where they are far apart.
• Acts perpendicular to isobars.

2. Frictional force

• Affects wind speed; greatest at the surface, influence up to 1-3 km; minimal over the sea.

3. Coriolis force

• Caused by the earth's rotation; named after the French physicist who described it in 1844.
• Deflects wind to the right in the northern hemisphere and left in the southern.
• Deflection is greater when wind velocity is higher.
• Directly proportional to the angle of latitude: maximum at the poles, absent at the equator.
• Acts perpendicular to the pressure gradient force.

Consequences:

• Because the two forces act at right angles, wind blows around a low.
• At the equator, Coriolis force is zero, wind blows perpendicular to isobars, and the low fills up instead of intensifying: that is why tropical cyclones do not form near the equator.

Geostrophic wind (Figure 9.4): in the upper air, 2-3 km up, free of friction, with straight isobars, the pressure gradient force is balanced by the Coriolis force and the wind blows parallel to the isobars.

Worked example

In the southern hemisphere, air flows from the subtropical high near 30°S towards the equatorial low. From which direction will the surface wind blow, and why? (2 marks)

Try it first, then show the solutionSolution

1. The air starts moving north, from the subtropical high towards the equatorial low.
2. The Coriolis force deflects it to the left in the southern hemisphere, so it blows from the south-east: the south-east trades.

Answer: From the south-east, because the Coriolis force deflects northward-moving air to the left.

Cyclonic and anticyclonic circulation; convergence and divergence

NCERT section Pressure and Wind

System	Centre	Northern hemisphere	Southern hemisphere
Cyclone	Low	Anticlockwise	Clockwise
Anticyclone	High	Clockwise	Anticlockwise

(Table 9.2)

• Circulation around a low = cyclonic; around a high = anticyclonic.
• Surface and upper-level circulations are often closely linked.
• Over a low, air converges and rises; over a high, air subsides from above and diverges at the surface (Figure 9.5).
• Besides convergence, eddies, convection currents, orographic uplift and uplift along fronts make air rise, which is essential for clouds and precipitation.

Worked example

Why do lows usually bring cloudy, rainy weather and highs clear weather? (2 marks)

Try it first, then show the solutionSolution

1. Over a low, air converges and rises; rising air cools and forms clouds and precipitation.
2. Over a high, air subsides from above and diverges at the surface, so air does not rise and skies stay clear.

Answer: Lows: rising air forms clouds; highs: sinking air keeps skies clear.

General circulation of the atmosphere and its effect on oceans

NCERT section Pressure and Wind

Planetary winds depend on: (i) latitudinal variation in heating; (ii) pressure belts; (iii) migration of belts with the sun's apparent path; (iv) distribution of continents and oceans; (v) the earth's rotation. Their pattern is the general circulation (Figure 9.6), which also drives ocean circulation and influences climate.

Three cells:

Cell	Where	How it works	Surface winds
Hadley	Tropics	Air at the ITCZ rises by convection (high insolation, low pressure), reaches the top of the troposphere (up to 14 km), moves poleward, accumulates near 30°N/S; part sinks (also because it cools) forming the subtropical high	Easterlies flowing back to the ITCZ
Ferrel	Middle latitudes	Sinking cold air from the poles and rising warm air from the subtropical high	Westerlies
Polar	Polar latitudes	Cold dense air subsides near the poles	Polar easterlies towards mid-latitudes

The transfer of heat from low to high latitudes maintains the general circulation.

Effect on oceans: large-scale winds start large, slow ocean currents; oceans in turn supply energy and water vapour to the air.

El Nino and ENSO:

• El Nino: warm water of the central Pacific drifts towards the South American coast and replaces the cool Peruvian current off Peru.
• Southern Oscillation: associated pressure changes over the central Pacific and Australia.
• ENSO = El Nino + Southern Oscillation. In strong ENSO years: heavy rain on the arid west coast of South America, drought in Australia and sometimes India, floods in China. It is monitored for long-range forecasting.

Worked example

Give two reasons for the formation of subtropical highs at about 30°N and 30°S. (2 marks)

Try it first, then show the solutionSolution

1. Air rising at the ITCZ moves poleward at the top of the troposphere and accumulates around 30°N and S; part of it sinks.
2. The air also cools by the time it reaches 30°N and S, becomes denser and sinks, forming high pressure at the surface.

Answer: Accumulation of poleward-moving upper air, and its cooling and sinking near 30°.

Seasonal and local winds

NCERT section Pressure and Wind

Seasonal winds: wind patterns change with seasons as regions of maximum heating, pressure and wind belts shift. The most pronounced example is the monsoon, especially over south-east Asia (detailed in India: Physical Environment).

Local winds arise from differences in heating and cooling of surfaces, with daily or annual cycles:

• Sea breeze (day): land heats faster, air over it rises, creating low pressure; the cooler sea has relatively high pressure, so wind blows from sea to land.
• Land breeze (night): land loses heat faster and becomes cooler than the sea; the gradient reverses and wind blows from land to sea (Figure 9.7).
• Valley breeze (day): slopes heat up, air moves upslope, and air from the valley blows up the valley to fill the gap.
• Mountain wind (night): slopes cool, and dense air descends into the valley.
• Katabatic wind: cool air of high plateaus and ice fields draining into the valley.
• Warm leeward wind: moisture condenses and falls as the wind crosses a range; descending the leeward slope, the dry air warms adiabatically and may melt snow quickly.

Worked example

Why does a sea breeze blow by day and a land breeze by night? (2 marks)

Try it first, then show the solutionSolution

1. By day land heats faster than the sea; low pressure over land and relatively high pressure over the sea make wind blow from sea to land.
2. At night land cools faster than the sea; pressure becomes higher over land, so wind blows from land to sea.

Answer: Different heating and cooling rates of land and sea reverse the pressure gradient between day and night.

Air masses and fronts

NCERT section Pressure and Wind

Air mass: when air stays over a homogeneous area (vast ocean or plain) long enough, it takes on that area's characteristics. It is a large body of air with little horizontal variation in temperature and moisture. The homogeneous surfaces are source regions.

Source region	Air mass
Warm tropical and subtropical oceans	Maritime tropical (mT)
Subtropical hot deserts	Continental tropical (cT)
Relatively cold high-latitude oceans	Maritime polar (mP)
Very cold snow-covered continents in high latitudes	Continental polar (cP)
Permanently ice-covered continents (Arctic, Antarctica)	Continental arctic (cA)

Tropical air masses are warm; polar ones cold.

Fronts: the boundary zone where two different air masses meet; their formation is frontogenesis.

Front	Situation
Cold	Cold air moves towards warm air
Warm	Warm air moves towards cold air
Stationary	The front does not move
Occluded	An air mass is fully lifted above the land surface

Fronts occur in middle latitudes, have steep gradients of temperature and pressure, bring abrupt temperature changes and make air rise to form clouds and precipitation (Figure 9.8).

Worked example

Winter air that reaches north India from Central Asia is very cold and dry. Name the kind of source region and air mass involved, and explain why the Himalayas or a hilly plateau could not be a source region. (2 marks)

Try it first, then show the solutionSolution

1. Air masses form over vast, homogeneous surfaces such as warm or cold oceans and extensive plains; a snow-covered high-latitude plain such as Siberia produces continental polar (cP) air, cold and dry.
2. Mountains and broken plateaus are not uniform over a large area, so air over them cannot acquire uniform temperature and humidity.

Answer: A cold continental plain (like the Siberian plain) giving continental polar (cP) air; uneven terrain cannot give air uniform properties.

Extra-tropical and tropical cyclones

NCERT section Pressure and Wind

Extra-tropical (middle latitude) cyclones

• Develop beyond the tropics, in mid and high latitudes, along the polar front.
• Stages: at first the front does not move; north of the equator, warm air comes from the south side of it and cold air from the north side. Pressure drops along the front; warm air moves north, cold air south, starting an anticlockwise circulation; a well-developed cyclone has a warm front and a cold front with a warm sector wedged between cold sectors (Figure 9.9).
• Warm air glides over cold air, producing a sequence of clouds and precipitation ahead of the warm front; the cold front pushes warm air up, forming cumulus clouds; the cold front moves faster, overtakes the warm front, the warm air is fully lifted, the front is occluded and the cyclone dissipates.

Tropical cyclones

• Violent storms originating over tropical oceans, moving to coasts with violent winds, very heavy rain and storm surges; among the most devastating calamities.
• Names: Cyclones (Indian Ocean), Hurricanes (Atlantic), Typhoons (Western Pacific, South China Sea), Willy-willies (Western Australia).
• Favourable conditions: (i) large sea surface above 27°C; (ii) Coriolis force; (iii) small variations in vertical wind speed; (iv) a pre-existing weak low or low-level cyclonic circulation; (v) upper divergence above the sea-level system.
• Energy: condensation in towering cumulonimbus clouds around the centre; moisture from the sea sustains it; on land the supply is cut and it dissipates. Landfall = where it crosses the coast. Cyclones crossing 20°N generally recurve and are more destructive.
• Structure (Figure 9.10): spiralling winds round a calm eye of subsiding air; eye wall with strong spiralling ascent up to the tropopause, winds up to 250 km/h, torrential rain; rain bands and cumulus/cumulonimbus clouds outward. Circulating system 150-250 km across; storms over the Bay of Bengal, Arabian Sea and Indian Ocean 600-1,200 km; move 300-500 km per day.

	Extra-tropical	Tropical
Fronts	Clear frontal system	None
Origin	Land and sea	Only over seas; dissipate on land
Area	Larger	Smaller
Wind velocity	Lower	Much higher, more destructive
Movement	West to east	East to west

Worked example

Why do tropical cyclones not form near the equator, even though the sea there is warm? (2 marks)

Try it first, then show the solutionSolution

1. Coriolis force is a required condition; it is zero at the equator.
2. Without it, wind blows straight into the low perpendicular to the isobars and fills it, instead of circling and intensifying it.

Answer: No Coriolis force at the equator, so the low fills up instead of deepening.

Thunderstorms and tornadoes

NCERT section Pressure and Wind

Severe local storms: short duration, small area, but violent.

Thunderstorms:

• Caused by intense convection on hot, moist days.
• It is a fully grown cumulonimbus cloud that gives thunder and lightning.
• If clouds reach heights with sub-zero temperatures, hail forms and falls as a hailstorm.
• With insufficient moisture, a thunderstorm can generate dust-storms.
• Intense updraft of rising warm air makes clouds grow bigger and higher, causing precipitation; later a downdraft brings cool air and rain to the ground.

Tornadoes:

• From severe thunderstorms, spiralling wind descends like an elephant's trunk, with great force and very low pressure at the centre, causing massive destruction.
• Generally occur in middle latitudes.
• Over the sea: waterspouts.

Meaning: these storms show the atmosphere adjusting to uneven energy distribution: potential and heat energy become kinetic energy, and the atmosphere returns to a stable state.

Worked example

When does a thunderstorm produce a hailstorm, and when a dust-storm? (2 marks)

Try it first, then show the solutionSolution

1. If the cumulonimbus cloud grows to heights with sub-zero temperatures, hail forms and falls as a hailstorm.
2. If there is not enough moisture, the thunderstorm can generate a dust-storm instead.

Answer: Hail when clouds reach sub-zero levels; dust-storm when moisture is insufficient.

Atmospheric Circulation and Weather Systems in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Air Masses, Fronts, Cyclones, Thunderstorms and Tornadoes
• Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds

Atmospheric Circulation and Weather Systems: key facts

Sea-level pressure

Average 1,013.2 mb (standard table 1,013.25 mb)

Measured by mercury or aneroid barometer.

Vertical pressure change

Fall ≈ 1 mb per 10 m in the lower atmosphere

Not always at the same rate (Table 9.1).

Reducing to sea level

Sea-level pressure ≈ station pressure + station height (m) ÷ 10

Uses the 1 mb per 10 m approximation.

Pressure belts

1. 1.Equatorial low (0°)
2. 2.subtropical high (30°)
3. 3.sub-polar low (60°)
4. 4.polar high (90°)

Shift north in northern summer, south in winter.

Coriolis force

Right in NH, left in SH; ∝ latitude; maximum at poles, zero at equator

Described in 1844.

Cyclone directions

Low: anticlockwise NH, clockwise SH | High: clockwise NH, anticlockwise SH

Table 9.2.

Three cells

Hadley	easterlies
Ferrel	westerlies
Polar	polar easterlies

ITCZ air rises to about 14 km.

Air masses

$mT,cT,mP,cP,cA$

m = maritime, c = continental; T = tropical, P = polar, A = arctic.

Tropical cyclone figures

SST > 27°C; eye-wall winds up to 250 km/h; system 150-250 km; Indian seas 600-1,200 km; moves 300-500 km/day

Recurve after crossing 20°N.

Atmospheric Circulation and Weather Systems: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Atmospheric Circulation and Weather Systems: map work

The places on the official CBSE map list for this chapter. Find each one on an outline map of India and practise marking it.

How the school exam asks Atmospheric Circulation and Weather Systems

The question patterns to expect, and the method that works for each.

Tricks to remember Atmospheric Circulation and Weather Systems

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Pressure belts

  'Low-High-Low-High' from equator to pole at 0°, 30°, 60°, 90°.

  Use it when: World pressure belts.

• Three forces

  'PFC': Pressure gradient pushes, Friction slows, Coriolis curves.

  Use it when: Factors affecting wind.

• Cyclone conditions

  'Warm Coriolis Calm Low Divergence': SST > 27°C, Coriolis, small vertical wind shear, pre-existing low, upper divergence.

  Use it when: Tropical cyclone conditions.

• Cyclone names

  'Indian Cyclone, Atlantic Hurricane, Pacific Typhoon, Australian Willy-willy': I-C, A-H, P-T, A-W.

  Use it when: Regional names.

• Fronts

  'Cold chases, Warm climbs, Stationary sits, Occluded overtaken'.

  Use it when: Types of fronts.

Atmospheric Circulation and Weather Systems: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Atmospheric Circulation and Weather Systems

The slips examiners see most often, and what to write instead.

• ✗Mistake: Reversing cyclone directions

  Clockwise and anticlockwise get swapped between hemispheres.

  Wrong working

  Cyclones rotate clockwise in the northern hemisphere.

  Right working

  Cyclones (lows) are anticlockwise in the NH and clockwise in the SH; anticyclones the reverse.

  ✓Do this: North Low = anticlockwise (both 'N' and 'A' look 'pointy').

  Usually costs: 1 mark per MCQ.

• ✗Mistake: Saying Coriolis force is strongest at the equator

  Rotation speed is highest at the equator, so students assume the force is too.

  Wrong working

  Coriolis force is maximum at the equator.

  Right working

  Coriolis force is proportional to latitude: maximum at the poles, zero at the equator.

  ✓Do this: No Coriolis, no cyclone at the equator.

  Usually costs: 1-2 marks.

• ✗Mistake: Mixing up the cells and their winds

  Three cells and three wind belts.

  Wrong working

  Ferrel cell gives the trade winds.

  Right working

  Hadley: easterlies (trades); Ferrel: westerlies; Polar: polar easterlies.

  ✓Do this: H-F-P from equator to pole.

  Usually costs: 1 mark per label.

• ✗Mistake: Writing that tropical cyclones move west to east

  Confusion with mid-latitude systems.

  Wrong working

  Tropical cyclones move from west to east.

  Right working

  Extra-tropical cyclones move west to east; tropical cyclones east to west.

  ✓Do this: Tropics follow the easterlies.

  Usually costs: 1 mark.

• ✗Mistake: Placing the strongest winds in the eye

  The 'eye of the storm' sounds like the worst part.

  Wrong working

  Winds are strongest in the eye.

  Right working

  The eye is calm with subsiding air; the strongest winds (up to 250 km/h) and torrential rain are in the eye wall.

  ✓Do this: Eye calm, wall wild.

  Usually costs: 1-2 marks.

NCERT exercise guide: Atmospheric Circulation and Weather Systems

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Atmospheric Circulation and Weather Systems

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   A weather map of the northern hemisphere shows a closed low with isobars of 996, 1000 and 1004 mb. If you stand with your back to the wind, the low will be on your left. Which way does the air circulate around the low?
   (a) Clockwise
   (b) Anticlockwise
   (c) Straight into the centre
   (d) Parallel to the equator

   Hint

   Coriolis deflects to the right in the northern hemisphere.

   Answer

   (b) Anticlockwise.

2. Q2 · 1 mark · MCQExam level

   Continental polar (cP) air masses form over:
   (a) warm tropical oceans
   (b) subtropical hot deserts
   (c) very cold snow-covered high-latitude continents
   (d) cold high-latitude oceans

   Hint

   c = continental, P = polar.

   Answer

   (c).

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): We do not experience strong upward winds.
   Reason (R): The vertical pressure gradient force is smaller than the horizontal one.
   Choose (a)-(d).

   Hint

   Which force balances the vertical one?

   Answer

   (c). A is true; R is false: the vertical force is much larger but is balanced by gravity.

4. Q4 · 3 marks · Short answerHarder exam level

   Using Table 9.1, find the average fall in pressure per km between 1 km and 5 km, and compare it with the fall between sea level and 1 km. What does this show? (3 marks)

   Hint

   898.76 - 540.48 over 4 km.

   Answer

   1-5 km: (898.76 - 540.48) ÷ 4 = 89.57 mb per km. Sea level-1 km: 1,013.25 - 898.76 = 114.49 mb. Pressure falls fastest near the surface and the rate decreases with height, so it does not fall at the same rate.

5. Q5 · 2 marks · Short answerExam level

   What are geostrophic winds? (2 marks)

   Hint

   Upper air, straight isobars.

   Answer

   Winds 2-3 km above the surface, free from friction, where with straight isobars the pressure gradient force is balanced by the Coriolis force, so the wind blows parallel to the isobars.

6. Q6 · 4 marks · Long answerHarder exam level

   Distinguish between extra-tropical and tropical cyclones on any four points. (4 marks)

   Hint

   Fronts, origin, area, speed, direction.

   Answer

   Extra-tropical: clear frontal system; form over land and sea; cover a larger area; lower wind velocity; move west to east. Tropical: no fronts; form only over seas and dissipate on land; smaller area; much higher wind velocity and more destructive; move east to west.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a skier in a mountain valley finds the snow on the leeward slopes melting fast on a dry, sunny day while a strong warm wind blows down the slope. Explain using the chapter. (3 marks)

   Hint

   Adiabatic warming on the leeward side.

   Answer

   The wind lost its moisture as precipitation while crossing the range on the windward side. Descending the leeward slope, the dry air warms adiabatically. This warm, dry wind can melt snow in a short time.

8. Q8 · 5 marks · Long answerHarder exam level

   Explain the life cycle of an extra-tropical cyclone. (5 marks)

   Hint

   Stationary front → circulation → warm and cold fronts → occlusion.

   Answer

   It starts along the polar front, initially stationary, with warm air from the south and cold air from the north. Pressure falls along the front; warm air moves north and cold air south, starting anticlockwise circulation. A warm front and cold front develop, with a warm sector between cold sectors. Warm air glides over cold air ahead of the warm front, giving clouds and rain; the cold front lifts warm air, forming cumulus clouds. The faster cold front overtakes the warm front, the warm air is fully lifted (occlusion) and the cyclone dissipates.

9. Q9 · 2 marks · Short answerHOTS / competency

   A cyclone is reported to be moving north-west across the Bay of Bengal and has just crossed 20°N. What does the chapter suggest about its track and danger? (2 marks)

   Hint

   Recurving.

   Answer

   Cyclones that cross 20°N generally recurve (change direction) and are more destructive, so the threat may increase and its path may shift.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is the average sea-level pressure?Show answer

   1,013.2 mb.

2. 2.Up to what height does friction affect wind?Show answer

   About 1-3 km.

3. 3.In which direction does Coriolis force deflect winds in the southern hemisphere?Show answer

   To the left.

4. 4.What is the cell in the tropics called?Show answer

   The Hadley cell.

5. 5.What is a katabatic wind?Show answer

   Cool air of high plateaus and ice fields draining into a valley.

6. 6.What is frontogenesis?Show answer

   The process of formation of fronts.

7. 7.What are tropical cyclones called in Western Australia?Show answer

   Willy-willies.

8. 8.What is a tornado over the sea called?Show answer

   A waterspout.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Pressure = weight of the air column; 1,013.2 mb at sea level; falls about 1 mb per 10 m.
• ☐Isobars after reduction to sea level; close isobars = strong gradient.
• ☐Belts: equatorial low, subtropical highs (30°), sub-polar lows (60°), polar highs; shift with the sun.
• ☐Forces: pressure gradient, friction (to 1-3 km), Coriolis (right NH, left SH; zero at equator).
• ☐Geostrophic wind: 2-3 km up, parallel to straight isobars.
• ☐Low: anticlockwise NH; high: clockwise NH. Lows converge and rise; highs subside and diverge.
• ☐Cells: Hadley (easterlies), Ferrel (westerlies), polar (polar easterlies); ITCZ air rises to 14 km.
• ☐El Nino + Southern Oscillation = ENSO; rain in Peru, drought in Australia and sometimes India, floods in China.
• ☐Local winds: land/sea breezes, valley/mountain winds, katabatic, warm leeward winds.
• ☐Air masses mT, cT, mP, cP, cA; fronts cold, warm, stationary, occluded.
• ☐Tropical cyclones: SST > 27°C, Coriolis, eye and eye wall (250 km/h), 300-500 km/day; extra-tropical move west to east.
• ☐Map: Mojave, Patagonian, Sahara, Gobi, Thar, Great Victoria.

Atmospheric Circulation and Weather Systems: frequently asked questions