This MCQ module is based on: Air Masses, Fronts, Cyclones & Exercises
TOPIC 19 OF 29
Air Masses, Fronts, Cyclones & Exercises
🎓 Class 11
Social Science
CBSE
Theory
Ch 9 — Atmospheric Circulation and Weather Systems
⏱ ~28 min
🌐 Language: [gtranslate]
🧠 AI-Powered MCQ Assessment
▲
📝 Worksheet / Assessment
▲
This assessment will be based on: Air Masses, Fronts, Cyclones & Exercises
Upload images, PDFs, or Word documents to include their content in assessment generation.
9.10 Air Masses — Giant Bodies of Uniform Air
So far we have looked at how pressure differences set air in motion. But the air itself is not uniform around the world. When a vast body of air sits over a homogenous region — say a giant tropical ocean or a snow-covered Arctic continent — for a sufficiently long time, it gradually takes on the temperature and humidity properties of the surface beneath it. Such a body is called an air mass?.
📖 Definition — Air Mass
An air mass is a large body of air having little horizontal variation in temperature and moisture. The homogenous land or water surface over which an air mass forms is called its source region. Once formed, an air mass can travel thousands of kilometres carrying its original characteristics and modifying the weather of the lands it crosses.
The Five Major Source Regions
There are five major source regions on the planet — and from these we recognise five corresponding air-mass types:
| Source Region | Type of Air Mass | Symbol | Character |
|---|---|---|---|
| Warm tropical and subtropical oceans | Maritime Tropical | mT | Warm, very humid |
| Subtropical hot deserts | Continental Tropical | cT | Hot, very dry |
| Relatively cold high-latitude oceans | Maritime Polar | mP | Cold, moist |
| Very cold snow-covered continents in high latitudes | Continental Polar | cP | Cold, dry |
| Permanently ice-covered Arctic and Antarctic | Continental Arctic | cA | Bitterly cold, very dry |
🧊 Tropical vs. Polar — A Simple Rule
Tropical air masses (mT, cT) are warm; polar air masses (mP, cP, cA) are cold. The first letter (m / c) tells you the moisture: m = maritime (humid), c = continental (dry). So a "cP" arrival in north India in winter means cold, dry air sweeping down from Siberia.
9.11 Fronts — Where Air Masses Meet
When two different air masses meet, they do not simply mix. They keep their own identity and a sharp boundary zone develops between them, sloping gently like a tilted wall. This boundary zone is called a front?. The process of formation of fronts is called frontogenesis; the breakdown of fronts is frontolysis.
Fronts occur in the middle latitudes, where tropical and polar air masses regularly meet. They are characterised by steep gradients in temperature and pressure across just a few kilometres, and they bring abrupt changes in the weather. As warm air is forced to rise over cold air at a front, clouds form and precipitation falls.
Cold Front
Cold air mass advances and pushes a warm air mass upward — steep slope, cumulonimbus clouds, short heavy showers and thunderstorms.
Warm Front
Warm air mass slides up over a retreating cold air mass — gentle slope, layered stratiform clouds, prolonged steady rainfall.
Stationary Front
Neither air mass advances; the boundary remains parked in place for days, often producing extended cloudy and showery weather.
Occluded Front?
Faster-moving cold front overtakes the slow warm front, fully lifting the warm air aloft — marks the dying stage of an extra-tropical cyclone.
Vertical Sections of the Four Fronts
9.12 Extra-Tropical (Mid-Latitude) Cyclones
The pressure systems developing in the mid and high latitudes, beyond the tropics, are called extra-tropical cyclones — also known as mid-latitude or frontal cyclones. They form along the polar front. The passage of a front causes abrupt changes in weather over middle and high latitudes.
Life Cycle of an Extra-Tropical Cyclone
- Stationary front stage. Cold polar easterlies meet warm westerlies along a stationary polar front. Warm air sits to the south, cold to the north (Northern Hemisphere).
- Wave stage. Pressure drops along the front. Warm air pushes north; cold air slides south. A wave-like kink develops on the front and an anticlockwise (cyclonic) circulation begins.
- Mature stage. A well-developed cyclone now has a warm sector wedged between a warm front (ahead) and a cold front (behind). Layered clouds and steady rain run ahead of the warm front; cumulus and thunderstorms cluster along the cold front.
- Occlusion stage. The cold front, which is faster, overtakes the warm front and lifts the warm sector clean off the ground — creating an occluded front. Surface temperature contrast disappears.
- Dissipation. Without warm-sector fuel the cyclone weakens and dies.
Plan View of a Mature Extra-Tropical Cyclone (Northern Hemisphere)
Extra-Tropical vs. Tropical Cyclones — A Comparison
| Feature | Extra-Tropical Cyclone | Tropical Cyclone |
|---|---|---|
| Frontal system | Yes — clear warm and cold fronts | No fronts |
| Origin | Over both land and sea | Only over warm seas (> 27 °C) |
| Area covered | Larger | Smaller, but more concentrated |
| Wind velocity | Lower | Much higher — up to 250 km/h |
| Direction of movement | West to East | East to West |
| Behaviour at landfall | Continues to travel | Quickly dissipates |
9.13 Tropical Cyclones — The Most Devastating Storms on Earth
Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas, bringing about large-scale destruction caused by violent winds, very heavy rainfall and storm surges. They are one of the most devastating natural calamities. Different oceans give them different names: Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the western Pacific and South China Sea, and Willy-willies in western Australia.
Five Conditions for Formation
Tropical cyclones originate and intensify only over warm tropical oceans. Five conditions are needed:
- A large sea surface with temperature higher than 27 °C — provides the moisture and latent-heat fuel.
- Presence of the Coriolis force — to give the storm rotational spin (so cyclones do not form within ~5° of the equator).
- Small variations in vertical wind speed (low wind shear) — so the rising column is not torn apart.
- A pre-existing weak low-pressure area or low-level cyclonic circulation — the seed.
- Strong upper-air divergence above the sea-level system — to vent the rising air outward at the top.
⚡ The Energy Source
The energy that intensifies a tropical cyclone comes from the condensation process inside the towering cumulonimbus clouds surrounding the centre. Each kilogram of water vapour that condenses releases about 2,500 kJ of latent heat. With continuous moisture supplied by the warm sea, the storm strengthens. The instant the cyclone moves over land its moisture supply is cut off — and it dies. The point where a tropical cyclone crosses the coast is called the landfall of the cyclone. Cyclones that cross 20° N latitude generally recurve, and these are the most destructive.
The Anatomy of a Mature Tropical Cyclone
A mature tropical cyclone is characterised by a strong, spirally circulating wind around a central calm zone called the eye?. The diameter of the circulating system varies between 150 and 250 km; the diameter of the storm field over the Bay of Bengal, Arabian Sea and Indian Ocean lies between 600 and 1,200 km. The system moves slowly, only 300–500 km per day.
Eye
Calm central region of subsiding air, clear skies, light winds. 30–60 km across.
Eye Wall
Ring of strongly rising air around the eye. Maximum wind velocity, up to 250 km/h. Torrential rainfall.
Spiral Rain Bands
Trains of cumulus and cumulonimbus clouds radiating outward from the eye wall — drift into the outer region.
Storm Surge
Sea water piled up against the coast by cyclonic winds — inundates low coastal lands. The biggest killer in Bay-of-Bengal cyclones.
Vertical Section of a Mature Tropical Cyclone
From the eye wall outward, spiral rain bands radiate, made up of cumulus and cumulonimbus clouds that drift slowly into the surrounding region. As the cyclone hits the coast it pushes a wall of sea water — the storm surge — that inundates low-lying coastal lands. The storm peters out on land. Indian cyclones such as the 1999 Odisha super-cyclone, the 2008 cyclone Nargis (Myanmar), the 2013 Phailin and the 2020 Amphan have repeatedly underlined the destructive power of these systems for the Bay of Bengal coast.
LET'S EXPLORE — Why Cyclones Die Over Land
L4 Analyse
Track the path of any recent Bay-of-Bengal cyclone (e.g. Phailin, Hudhud, Fani, Amphan, Yaas, Mocha) using a satellite or atlas. Note how rapidly the cyclone weakens once it makes landfall. Explain in your own words why a tropical cyclone needs the sea — and what happens to its energy supply on land.
A tropical cyclone is essentially a giant heat engine that runs on water-vapour fuel. Warm sea-water (> 27 °C) evaporates into rising air; high in the storm the vapour condenses and releases latent heat, which warms the air column further and lowers the surface pressure — feeding the storm. Once the cyclone moves over land, two things happen: (i) the moisture supply is cut off, depriving the engine of fuel; (ii) the rough land surface adds friction, slowing the winds. With no new latent heat, the central pressure rises and the cyclone collapses within 24–48 hours. This is why coastal districts of Odisha and Andhra Pradesh take the heaviest blow but Madhya Pradesh rarely sees cyclonic winds.
9.14 Thunderstorms and Tornadoes — Short, Violent and Local
Other severe local storms are thunderstorms and tornadoes. They are of short duration, occur over a small area but are violent.
Thunderstorms
Thunderstorms are caused by intense convection on moist hot days. A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightning. When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as a hailstorm. If there is insufficient moisture, a thunderstorm can generate a dust-storm. The storm is characterised by an intense updraft of rising warm air, which causes the cloud to grow to greater height. This causes precipitation. Later, a downdraft brings cool air and rain back to earth.
Tornadoes
From severe thunderstorms, sometimes a spiralling column of wind descends from the cloud base like the trunk of an elephant, with great force and very low pressure at the centre, causing massive destruction along its path. This is a tornado. Tornadoes generally occur in middle latitudes — the central United States ("Tornado Alley") records the highest frequency on the planet. A tornado over the sea is called a water spout.
🌬️ The Atmosphere Re-balances
These violent storms — cyclones, thunderstorms, tornadoes — are the manifestation of the atmosphere's adjustments to varying energy distribution. Potential and heat energies are converted into kinetic energy in these storms, and the restless atmosphere again returns to its stable state. Behind every calm sky there is a planet-scale settling of accounts.
SOURCE — From the NCERT Textbook
L2 Understand
NCERT writes: "These violent storms are the manifestation of the atmosphere's adjustments to varying energy distribution. The potential and heat energies are converted into kinetic energy in these storms and the restless atmosphere again returns to its stable state." Explain this idea using one example, naming the form of energy at each stage.
Take a tropical cyclone over the Bay of Bengal. The starting form of energy is solar (radiative) energy, which heats the sea surface — this is potential energy stored in warm seawater. Evaporation converts that surface heat into latent heat stored in water vapour. As the moist air rises into the cyclone, condensation releases the latent heat, warming the air column and converting energy into the heat-and-pressure imbalance that drives the storm. The pressure imbalance accelerates the air — converting heat energy into kinetic energy (motion) that we feel as 200-km/h winds. Once the storm hits the coast it loses its moisture source; the kinetic energy is dissipated by friction and the system collapses. The atmosphere has redistributed energy from the warm tropical sea to the cooler land, and returned to its stable state.
🎯 Competency-Based Questions — Air Masses, Fronts & Cyclones
Case Stem. The India Meteorological Department issues a bulletin on 12 May: "A deep depression over the south-east Bay of Bengal is intensifying into a Severe Cyclonic Storm. Sea surface temperature 30 °C. Vertical wind shear weak. Central pressure 982 mb, expected to fall further. Likely landfall on the Odisha–West Bengal coast in 36 hours, with winds up to 180 km/h and a 4-m storm surge." Use the bulletin to answer Q1–Q4.
Q1. Which two NCERT-listed conditions are explicitly satisfied in the bulletin for cyclone intensification?
L3 ApplyAnswer: (a). NCERT lists (i) SST > 27 °C and (ii) small variations in vertical wind speed (low shear) among the five conditions for cyclone formation. Both are present here. Coriolis is non-zero in the Bay (latitude ~12° N), so option (b) is wrong.
Q2. The bulletin warns of a 4-m storm surge. Why is the storm surge often more lethal than the wind itself in Bay-of-Bengal cyclones?
L4 AnalyseBecause the Bay of Bengal coast is unusually low and wide. The funnel-shape of the Bay concentrates storm-driven seawater against the deltaic coasts of Bangladesh and Odisha. A storm surge inundates dozens of kilometres of low-lying farmland and villages, carrying salt water, debris and contamination. The 1970 Bhola cyclone killed an estimated 300,000 people — almost all by drowning, not wind.
Q3. After landfall, the bulletin predicts the cyclone will weaken into a "depression" within 24 hours. Justify this forecast in physical terms.
L5 EvaluateBecause the land cuts off the moisture-and-latent-heat supply. The cyclone runs on energy released when sea-water vapour condenses inside the eye-wall clouds. On land there is no warm ocean to feed it, and surface friction is much greater. The condensation engine starves, the central pressure rises, and within 24–48 hours the storm becomes a depression and dies. Extra-tropical cyclones, in contrast, get their energy from front-zone temperature contrasts and can keep travelling overland.
HOT Q. Design a 5-step preparedness checklist for a coastal village in Odisha 36 hours before cyclone landfall, justifying each step with one fact from the chapter.
L6 CreateSample checklist. (1) Move all residents within 5 km of the coast to inland concrete cyclone shelters — the storm surge can inundate up to 5 km of low coast. (2) Secure roofs and uproot loose objects — winds may exceed 180 km/h. (3) Stockpile drinking water and dry food for 48 hours — power and roads will be cut. (4) Cut off external electricity to prevent fires from fallen lines. (5) Evacuate fishermen back to harbour — the storm field over the Indian Ocean is 600–1,200 km wide and rough seas will occur far ahead of landfall. Each step maps to a chapter fact.
⚖️ Assertion–Reason Questions — Class 11
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): Tropical cyclones dissipate quickly after landfall, while extra-tropical cyclones often continue to travel overland.
Reason (R): Tropical cyclones are powered by latent heat from warm ocean evaporation, while extra-tropical cyclones are powered by horizontal temperature contrasts along fronts.
Answer: (A) — Both true and R correctly explains A. Without an ocean source, the latent-heat engine of a tropical cyclone shuts off; an extra-tropical cyclone, by contrast, can keep tapping front-zone temperature gradients across continents.
Assertion (A): The eye of a mature tropical cyclone is a region of calm and clear sky.
Reason (R): The eye is a column of subsiding air at the centre of the storm; sinking air warms and dries, suppressing cloud formation.
Answer: (A) — Both true and R is the correct explanation. Pilots flying through cyclones report that the eye is bizarrely calm — blue sky overhead, while the eye-wall just kilometres away packs the most violent winds on the planet.
Assertion (A): An occluded front marks the dying stage of an extra-tropical cyclone.
Reason (R): When the faster-moving cold front overtakes the slower warm front, the warm sector is fully lifted off the ground and the surface temperature contrast that fed the cyclone disappears.
Answer: (A) — Both true and R explains A. Once the warm sector has been occluded aloft, the front-zone temperature gradient at the surface is gone, and the cyclone's pressure-driven rotation soon weakens.
📝 NCERT Exercises — 1. Multiple Choice Questions
(i) If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:
(a) 700 mb (b) 1,100 mb (c) 900 mb (d) 1,300 mb
Answer: (c) 900 mb. Pressure decreases at roughly 1 mb for every 10 m of elevation gain in the lower atmosphere. So 1 km (= 1,000 m) gives a drop of about 100 mb, leaving 1,000 − 100 = 900 mb. Numbers (a) 700 mb and (b)/(d) (which would mean pressure rising with altitude) are physically impossible.
(ii) The Inter-Tropical Convergence Zone normally occurs:
(a) near the Equator (b) near the Tropic of Cancer (c) near the Tropic of Capricorn (d) near the Arctic Circle
Answer: (a) near the Equator. The ITCZ is the zone where the NE and SE trade winds converge. It coincides with the equatorial low-pressure belt and shifts a few degrees north or south of the geographical equator with the seasonal migration of the overhead Sun, but always stays close to the equator.
(iii) The direction of wind around a low pressure in the northern hemisphere is:
(a) clockwise (b) perpendicular to isobars (c) anti-clockwise (d) parallel to isobars
Answer: (c) anti-clockwise. A low (cyclone) in the Northern Hemisphere has converging air; the Coriolis force deflects each parcel to its right, producing an anticlockwise spiral around the centre when viewed from above. (In the Southern Hemisphere it would be clockwise.)
(iv) Which one of the following is the source region for the formation of air masses?
(a) the Equatorial forest (b) the Himalayas (c) the Siberian Plain (d) the Deccan Plateau
Answer: (c) the Siberian Plain. A source region must be a vast, homogenous land or water surface where air can stagnate long enough to acquire uniform temperature and humidity properties. The Siberian Plain is a huge, snow-covered, low-relief continental interior — a classic continental polar (cP) source. Equatorial forest is too narrow and humid-variable; the Himalayas and the Deccan Plateau are too rugged and small.
📝 NCERT Exercises — 2. Answer in About 30 Words
(i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?
Pressure is measured in millibar (mb), with 1 mb = 100 pascal (Pa). Average sea-level pressure is 1,013.25 mb. Station readings are reduced to mean sea level so that pressure differences from altitude are removed and only the horizontal pressure pattern shows up on the isobars — otherwise every hill station would falsely register as a low.
(ii) While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north-easterlies in the tropics?
Because of the Coriolis force. The pressure gradient pushes air southwards from the subtropical high (30° N) to the equatorial low. But the earth's rotation deflects this air to its right in the Northern Hemisphere, turning a southward-bound wind into one blowing from the north-east. That is why we call them the north-east trade winds rather than the "north winds".
(iii) What are the geostrophic winds?
Geostrophic winds are upper-air winds (above 2–3 km) that blow parallel to straight isobars. They occur where the pressure-gradient force is exactly balanced by the Coriolis force and friction is absent. Pilots and weather forecasters use the geostrophic balance to read upper-air pressure patterns straight from a chart.
(iv) Explain the land and sea breezes.
Land and sea breezes are daily local winds caused by the unequal heating of land and sea. By day the land heats faster, air rises over it and cooler air flows in from the sea — the sea breeze. By night the land loses heat faster, becomes cooler than the sea, and the wind reverses, blowing from land to sea — the land breeze. Coastal residents feel this rhythm every 24 hours.
📝 NCERT Exercises — 3. Answer in About 150 Words
(i) Discuss the factors affecting the speed and direction of wind.
The horizontal wind near the surface responds to a combined effect of three forces.
(1) Pressure-gradient force. The rate of change of pressure with horizontal distance. It pushes air from high to low pressure and is perpendicular to the isobars. Closely-packed isobars give a strong gradient and high wind speed.
(2) Frictional force. Acts opposite to wind motion. It is greatest at the surface, with influence reaching up to 1–3 km. Friction is small over the smooth ocean and large over rough land.
(3) Coriolis force. Caused by the earth's rotation. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern. It is zero at the equator and maximum at the poles, and grows with wind speed.
Together, these forces decide the velocity (mostly from the pressure gradient and friction) and the direction (modified by Coriolis). Above 2–3 km, where friction vanishes, the pressure-gradient and Coriolis forces balance to give the geostrophic wind, which blows parallel to the isobars.
(1) Pressure-gradient force. The rate of change of pressure with horizontal distance. It pushes air from high to low pressure and is perpendicular to the isobars. Closely-packed isobars give a strong gradient and high wind speed.
(2) Frictional force. Acts opposite to wind motion. It is greatest at the surface, with influence reaching up to 1–3 km. Friction is small over the smooth ocean and large over rough land.
(3) Coriolis force. Caused by the earth's rotation. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern. It is zero at the equator and maximum at the poles, and grows with wind speed.
Together, these forces decide the velocity (mostly from the pressure gradient and friction) and the direction (modified by Coriolis). Above 2–3 km, where friction vanishes, the pressure-gradient and Coriolis forces balance to give the geostrophic wind, which blows parallel to the isobars.
(ii) Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30° N and S latitudes?
The simplified diagram is the three-cell model shown earlier in this lesson — Hadley Cell (0–30°), Ferrel Cell (30–60°) and Polar Cell (60–90°) in each hemisphere, with surface wind belts of trade winds, westerlies and polar easterlies, and pressure belts of equatorial low, subtropical high, sub-polar low and polar high.
The subtropical high at 30° N and S forms for two reasons.
(a) Dynamic accumulation. Air rising at the equatorial low (ITCZ) reaches the top of the troposphere at ~14 km and drifts polewards. Around 30° latitude this poleward flow piles up because the Coriolis deflection turns it eastward (forming the sub-tropical jet), and part of the accumulated air sinks back to the surface.
(b) Cooling. By the time the air reaches 30° latitudes it has cooled at altitude, become denser, and tends to subside.
The descending dry air gives the subtropical high a dynamic origin and explains the world's great deserts (Sahara, Arabian, Thar, Australian) — all sitting under it.
The subtropical high at 30° N and S forms for two reasons.
(a) Dynamic accumulation. Air rising at the equatorial low (ITCZ) reaches the top of the troposphere at ~14 km and drifts polewards. Around 30° latitude this poleward flow piles up because the Coriolis deflection turns it eastward (forming the sub-tropical jet), and part of the accumulated air sinks back to the surface.
(b) Cooling. By the time the air reaches 30° latitudes it has cooled at altitude, become denser, and tends to subside.
The descending dry air gives the subtropical high a dynamic origin and explains the world's great deserts (Sahara, Arabian, Thar, Australian) — all sitting under it.
(iii) Why does a tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why?
Tropical cyclones can only form over warm seas because they need five conditions, three of which are oceanic.
(1) Sea-surface temperature above 27 °C — provides moisture and latent heat (the storm's fuel).
(2) Continuous evaporation supply, possible only over a wide ocean.
(3) Low vertical wind shear and an upper-air divergence zone.
The fourth and fifth conditions — Coriolis force and a pre-existing weak low — depend on latitude and synoptic conditions.
The energy that drives the storm comes from condensation in the towering cumulonimbus clouds. On reaching land the moisture supply is cut off and the cyclone dissipates.
Torrential rain and the highest wind velocities occur in the eye wall — the ring of strongly rising air that surrounds the calm central eye. In the eye wall the air rises explosively to the tropopause, condensing huge amounts of water vapour and releasing latent heat; wind speeds here can reach 250 km/h. Outside the eye wall, the spiral rain bands carry weaker but still heavy rain. The eye itself is calm because the air there is sinking — the eye-wall updraft is what concentrates both the rain and the wind.
(1) Sea-surface temperature above 27 °C — provides moisture and latent heat (the storm's fuel).
(2) Continuous evaporation supply, possible only over a wide ocean.
(3) Low vertical wind shear and an upper-air divergence zone.
The fourth and fifth conditions — Coriolis force and a pre-existing weak low — depend on latitude and synoptic conditions.
The energy that drives the storm comes from condensation in the towering cumulonimbus clouds. On reaching land the moisture supply is cut off and the cyclone dissipates.
Torrential rain and the highest wind velocities occur in the eye wall — the ring of strongly rising air that surrounds the calm central eye. In the eye wall the air rises explosively to the tropopause, condensing huge amounts of water vapour and releasing latent heat; wind speeds here can reach 250 km/h. Outside the eye wall, the spiral rain bands carry weaker but still heavy rain. The eye itself is calm because the air there is sinking — the eye-wall updraft is what concentrates both the rain and the wind.
🔬 Project Work
(i) Collect weather information over media such as newspaper, TV and radio for understanding the weather systems.
Suggested method. Make a 7-day chart with columns for date, max/min temperature, sky condition, wind direction, rainfall and pressure. Tick the source for each observation (newspaper, TV bulletin, IMD app, radio). At the end of the week, write a one-page note describing how the values changed and link them to the weather system shown on the IMD synoptic chart (e.g. a low approaching from the Bay, a western disturbance from the Mediterranean, a heat-wave anticyclone over the desert).
(ii) Read the section on weather in any newspaper, preferably one having a map showing a satellite picture. Mark the area of cloudiness. Attempt to infer the atmospheric circulation from the distribution of clouds. Compare the forecast given in the newspaper with the TV coverage, if you have access to TV. Estimate how many days in a week was the forecast accurate.
Sample observation method. Photocopy or trace the satellite cloud picture printed in a leading daily for seven consecutive days. With a pencil, shade the heaviest cloud bands. Look for: (i) a long curving cloud band over the Bay or Arabian Sea — likely a tropical depression; (ii) a comma-shaped cloud system over north India — likely an extra-tropical "western disturbance"; (iii) cumulus dotting central India in summer — convective thunderstorms. Mark the centre of any low or high. Then tabulate the newspaper's verbal forecast (e.g. "thundershowers expected") against what actually happened, and compute the hit-rate. Most students find a 5/7 to 6/7 success rate, with the largest errors in convective rainfall amount.
📚 Chapter 9 — Summary in 12 Points
- Atmospheric pressure is the weight of an air column from sea level to the top of the atmosphere; unit = millibar (mb), 1 mb = 100 Pa. Mean sea-level pressure = 1,013.25 mb.
- Measured by mercury barometer or aneroid barometer; pressure falls about 1 mb per 10 m of elevation in the lower atmosphere.
- Vertical pressure-gradient force is balanced by gravity, so we do not get strong upward winds.
- Isobars connect points of equal sea-level-reduced pressure; closed isobars define cyclones (low) and anticyclones (high).
- World pressure belts: Equatorial Low at 0°, Subtropical Highs at 30° N & S, Sub-polar Lows at 60° N & S, Polar Highs at 90°. Belts shift with the Sun.
- Three forces shape surface wind: pressure-gradient force (perpendicular to isobars), frictional force (within 1–3 km), Coriolis force (right in N hemisphere, left in S; zero at equator).
- Above 2–3 km, the pressure-gradient and Coriolis forces balance to give the geostrophic wind, which blows parallel to the isobars.
- Three-cell circulation: Hadley, Ferrel, Polar. Surface wind belts: NE/SE trades, westerlies, polar easterlies, with calm doldrums at 0° and horse latitudes at 30°.
- Monsoons are the most pronounced seasonal modification of the planetary winds. Local winds: Loo, Foehn, Chinook, Mistral, sea/land breezes, mountain/valley breezes (anabatic/katabatic).
- An air mass is a vast body of air with uniform temperature and humidity. Five types: mT, cT, mP, cP, cA. Their boundary is a front — cold, warm, stationary, or occluded.
- Extra-tropical cyclones form along the polar front, have clear fronts, move west-to-east, and last for days. Tropical cyclones form only over seas warmer than 27 °C, with a calm eye, a violent eye wall (winds up to 250 km/h), and spiral rain bands; system diameter 150–250 km, storm field 600–1,200 km. They die quickly on landfall.
- Thunderstorms are tall cumulonimbus clouds with thunder and lightning; tornadoes are spiralling columns of wind descending from severe thunderstorms in middle latitudes — a tornado over the sea is a water spout.
🗝️ Key Terms — Chapter 9
Atmospheric PressureWeight of an air column from sea level to the top of the atmosphere. Unit: mb (1,013.25 mb at sea level).
IsobarLine on a map joining places of equal sea-level pressure.
Pressure GradientRate of change of pressure with horizontal distance; drives wind from high to low.
Coriolis ForceApparent deflection caused by earth's rotation; right in N hemisphere, left in S; zero at equator (Coriolis, 1844).
Geostrophic WindUpper-air wind blowing parallel to straight isobars when pressure-gradient and Coriolis forces balance.
Trade WindsNE in N hemisphere, SE in S hemisphere; surface winds between subtropical high and equatorial low.
WesterliesMid-latitude (30°–60°) surface winds blowing from subtropical high to sub-polar low.
DoldrumsCalm zone at the equatorial low (ITCZ); feared by sailing ships.
Horse LatitudesCalm zone of the subtropical high at 30°.
MonsoonSeasonal reversal of winds caused by differential heating of land and sea — from Arabic mausim.
LooHot, dry westerly summer wind of north India and Pakistan.
FoehnWarm dry wind on the leeward (northern) side of the Alps.
Chinook"Snow-eater" — warm dry wind on the eastern side of the Rockies.
MistralCold dry north-westerly wind in southern France.
Air MassLarge body of air with uniform temperature and humidity; types mT, cT, mP, cP, cA.
FrontSloping boundary zone between two air masses: cold, warm, stationary or occluded.
CycloneLow-pressure system with converging spiral winds.
AnticycloneHigh-pressure system with diverging winds; clockwise in N hemisphere.
Occluded FrontFront formed when a fast cold front overtakes a slow warm front, lifting the warm sector aloft.
Eye WallRing of intensely rising air around the calm eye of a tropical cyclone; winds up to 250 km/h.
AI Tutor
Class 11 Geography — Fundamentals of Physical Geography
Ready