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Atmospheric Pressure and Winds

NCERT Class 11 · Geography Based on NCERT Class 11 Geography textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains the causes of pressure differences in the atmosphere, the forces that control wind movement, and the resulting global and local wind patterns. It covers the formation of air masses, fronts, and the development of both extra-tropical and tropical cyclones. The one big idea a student must walk away with is that the uneven heating of the Earth's surface creates pressure differences, which set the air in motion, and that this motion is modified by the Earth's rotation, friction, and the distribution of land and water, ultimately driving weather systems.
Key Concepts (31)
Atmospheric pressure
The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere.
e.g. At sea level, the average atmospheric pressure is 1,013.2 millibars, which is why we do not feel crushed by the air above us.
Millibar
The unit used to express atmospheric pressure.
e.g. Weather maps show pressure in millibars, e.g., 1,013.2 mb at sea level.
Isobar
Lines connecting places having equal pressure on a weather map.
e.g. On a weather map, closely spaced isobars indicate a strong pressure gradient and high wind speeds.
Pressure gradient force
The force produced by differences in atmospheric pressure; it is the rate of change of pressure with respect to distance.
e.g. When isobars are close together, the pressure gradient is strong, and winds blow faster.
Coriolis force
The force caused by the rotation of the Earth about its axis that deflects the wind to the right in the northern hemisphere and to the left in the southern hemisphere.
e.g. In the northern hemisphere, winds blowing from the subtropical high towards the equator are deflected to the right, becoming north-easterlies.
Frictional force
The force that affects the speed of the wind, greatest at the surface and extending up to an elevation of 1-3 km.
e.g. Over the sea, friction is minimal, so winds are stronger than over land at the same pressure gradient.
Geostrophic wind
A wind that blows parallel to the isobars when the pressure gradient force is balanced by the Coriolis force, occurring above the friction layer (2-3 km above the surface).
e.g. In the upper atmosphere, winds often blow parallel to straight isobars as geostrophic winds.
Cyclonic circulation
Wind circulation around a low-pressure system; in the northern hemisphere it is anticlockwise, and in the southern hemisphere it is clockwise.
e.g. A cyclone over the Bay of Bengal has anticlockwise wind circulation in the northern hemisphere.
Anticyclonic circulation
Wind circulation around a high-pressure system; in the northern hemisphere it is clockwise, and in the southern hemisphere it is anticlockwise.
e.g. A high-pressure system over Siberia in winter has clockwise wind circulation in the northern hemisphere.
Inter Tropical Convergence Zone (ITCZ)
The low-pressure zone near the equator where winds from the tropics converge and air rises due to convection caused by high insolation.
e.g. The ITCZ is responsible for heavy rainfall in the equatorial regions, such as over the Indian Ocean near the equator.
Hadley Cell
A tropical atmospheric circulation cell where air rises at the equator, moves poleward at high altitude, sinks at about 30° N and S, and flows back towards the equator as easterlies.
e.g. The Hadley Cell explains the formation of the subtropical high-pressure belts at 30° latitude.
Ferrel Cell
A mid-latitude atmospheric circulation cell where air rises from the subtropical high and moves poleward, sinking at the subpolar lows, with surface winds blowing as westerlies.
e.g. The Ferrel Cell is responsible for the prevailing westerlies in the middle latitudes.
Polar cell
A polar atmospheric circulation cell where cold, dense air subsides near the poles and blows towards middle latitudes as polar easterlies.
e.g. The polar cell creates the polar high-pressure areas and the polar easterlies.
Air mass
A large body of air having little horizontal variation in temperature and moisture, acquired by remaining over a homogeneous area for a sufficiently long time.
e.g. A maritime tropical (mT) air mass forms over warm tropical oceans and is warm and humid.
Front
The boundary zone between two different air masses when they meet.
e.g. When a cold air mass meets a warm air mass, the boundary is called a cold front.
Frontogenesis
The process of formation of fronts.
e.g. Frontogenesis occurs when two contrasting air masses are brought together by wind patterns.
Extra tropical cyclone
A cyclone that develops in the mid and high latitudes (beyond the tropics), characterized by a clear frontal system and moving from west to east.
e.g. The winter storms that affect the United Kingdom are extra tropical cyclones.
Tropical cyclone
A violent storm that originates over warm tropical oceans, characterized by strong spirally circulating wind around a central eye, with very heavy rainfall and storm surges.
e.g. Cyclone Amphan (2020) was a tropical cyclone that caused massive destruction in West Bengal and Bangladesh.
Eye
The centre of a mature tropical cyclone, a region of calm with subsiding air.
e.g. In a cyclone, the eye is the calm area surrounded by the violent eyewall.
Eye wall
The region around the eye of a tropical cyclone where there is strong spiralling ascent of air to great height, with maximum wind velocity and torrential rain.
e.g. The most destructive part of a cyclone is the eyewall, where wind speeds can reach 250 km per hour.
El Nino
The appearance of warm water off the coast of Peru, replacing the cool Peruvian current, associated with pressure changes in the central Pacific.
e.g. During an El Nino event, the west coast of South America receives heavy rainfall.
Southern Oscillation
The change in pressure conditions over the central Pacific and Australia associated with El Nino.
e.g. The Southern Oscillation Index measures the pressure difference between Tahiti and Darwin.
ENSO
The combined phenomenon of the Southern Oscillation and El Nino.
e.g. Strong ENSO events cause large-scale weather variations, such as droughts in Australia and floods in China.
Land breeze
A wind that blows from land to sea at night, when the land cools faster than the sea and creates a pressure gradient from land to sea.
e.g. In coastal cities like Chennai, a land breeze is felt at night, bringing cooler air from the land.
Sea breeze
A wind that blows from sea to land during the day, when the land heats up faster than the sea and creates a pressure gradient from sea to land.
e.g. In Mumbai, the sea breeze provides relief from the heat during the afternoon.
Valley breeze
A wind that blows upslope from the valley during the day when the slopes get heated up.
e.g. In the Himalayas, valley breezes are common during summer days.
Mountain wind
A wind that descends into the valley at night when the slopes get cooled and dense air descends.
e.g. Mountain winds are common in mountainous regions like the Alps.
Katabatic wind
The cool air of high plateaus and ice fields draining into the valley.
e.g. The katabatic winds in Antarctica are extremely cold and strong.
Thunderstorm
A severe local storm caused by intense convection on moist hot days, characterized by a well-grown cumulonimbus cloud producing thunder and lightning.
e.g. Summer afternoons in Kolkata often experience thunderstorms with heavy rain.
Tornado
A spiralling wind that descends from a severe thunderstorm like a trunk of an elephant with great force, with very low pressure at the centre, causing massive destruction.
e.g. Tornadoes are common in the central United States (Tornado Alley).
Water spout
A tornado that occurs over the sea.
e.g. Water spouts are sometimes seen off the coast of Florida.
How It Works
Vertical Variation of Pressure
  1. In the lower atmosphere, pressure decreases rapidly with height at a rate of about 1 mb for each 10 m increase in elevation.
  2. The vertical pressure gradient force is much larger than the horizontal pressure gradient force.
  3. This vertical force is generally balanced by an equal but opposite gravitational force, so we do not experience strong upward winds.
💡 Pressure does not always decrease at the same rate; the standard atmosphere table shows average values.
Formation of Geostrophic Wind
  1. Above the friction layer (2-3 km above the surface), the wind is free from frictional effects.
  2. The pressure gradient force acts perpendicular to the isobars, from high to low pressure.
  3. The Coriolis force acts perpendicular to the direction of wind, deflecting it to the right in the northern hemisphere.
  4. When the pressure gradient force is exactly balanced by the Coriolis force, the wind blows parallel to the isobars.
  5. This wind is called the geostrophic wind.
💡 Geostrophic wind only occurs when isobars are straight and friction is absent; it does not blow across isobars.
General Circulation of the Atmosphere (Three-Cell Model)
  1. At the equator, high insolation causes air to rise by convection, creating the Inter Tropical Convergence Zone (ITCZ) with low pressure.
  2. The rising air reaches the top of the troposphere (about 14 km) and moves poleward.
  3. At about 30° N and S, the air accumulates, cools, and sinks, forming subtropical high-pressure belts.
  4. Near the surface, the air flows back towards the equator as the easterlies (trade winds), completing the Hadley Cell.
  5. In the middle latitudes, warm air from the subtropical high moves poleward and meets cold air from the poles, rising at the subpolar lows (60° N and S) to form the Ferrel Cell.
  6. At the poles, cold dense air subsides, creating polar highs, and blows towards middle latitudes as polar easterlies, completing the Polar Cell.
💡 The three cells (Hadley, Ferrel, Polar) are not independent; they interact to transfer heat from the equator to the poles.
Formation of Extra Tropical Cyclones
  1. Extra tropical cyclones form along the polar front, where warm air from the south and cold air from the north meet.
  2. Initially, the front is stationary.
  3. When pressure drops along the front, the warm air moves northwards and the cold air moves southwards, setting in motion an anticlockwise cyclonic circulation in the northern hemisphere.
  4. A well-developed cyclone has a warm front and a cold front, with a warm sector wedged between two cold sectors.
  5. The warm air glides over the cold air ahead of the warm front, causing a sequence of clouds and precipitation.
  6. The cold front approaches from behind, pushing the warm air up and forming cumulus clouds.
  7. The cold front moves faster than the warm front, eventually overtaking it, lifting the warm air completely, and occluding the front.
  8. The cyclone then dissipates.
💡 Extra tropical cyclones have a clear frontal system, unlike tropical cyclones, and they move from west to east.
Formation and Intensification of Tropical Cyclones
  1. Tropical cyclones originate over warm tropical oceans with sea surface temperatures higher than 27°C.
  2. A pre-existing weak low-pressure area or low-level cyclonic circulation is required.
  3. The Coriolis force must be present (so they do not form near the equator).
  4. Small variations in vertical wind speed are necessary to allow the storm to intensify.
  5. Upper-level divergence above the sea-level system helps maintain the low pressure.
  6. Energy comes from condensation in towering cumulonimbus clouds surrounding the centre.
  7. Continuous moisture supply from the sea strengthens the storm.
  8. On reaching land, the moisture supply is cut off, and the storm dissipates.
💡 Tropical cyclones do not have frontal systems; they are characterised by an eye and an eyewall, and they move from east to west.
Land and Sea Breezes
  1. During the day, land heats up faster than the sea, creating a low-pressure area over land and relatively high pressure over the sea.
  2. A pressure gradient from sea to land is created, and wind blows from the sea to the land as a sea breeze.
  3. At night, land cools faster than the sea, creating a pressure gradient from land to sea.
  4. Wind blows from the land to the sea as a land breeze.
💡 Sea breezes occur during the day, land breezes at night; the direction reverses due to differential heating and cooling.
Mountain and Valley Winds
  1. During the day, mountain slopes get heated, causing air to move upslope.
  2. Air from the valley blows up the valley to fill the gap, creating a valley breeze.
  3. At night, the slopes cool, and dense air descends into the valley as a mountain wind.
  4. Cool air from high plateaus and ice fields draining into the valley is called katabatic wind.
💡 Valley breezes are daytime upslope winds; mountain winds are nighttime downslope winds.
Exam Traps — Don't Make These Mistakes
⚠ Confusing Cyclonic and Anticyclonic Circulation in the Southern Hemisphere
✗ In the southern hemisphere, winds around a low pressure are clockwise, and around a high pressure are anticlockwise.
→ This is actually correct, but many students write the opposite because they memorise the northern hemisphere pattern and forget to reverse it.
✓ In the southern hemisphere, cyclonic circulation (around a low) is clockwise, and anticyclonic circulation (around a high) is anticlockwise.
⚠ Mixing Up the Direction of Geostrophic Wind
✗ Geostrophic wind blows from high pressure to low pressure.
→ Geostrophic wind blows parallel to the isobars, not across them. The pressure gradient force is balanced by the Coriolis force, so the wind does not cross isobars.
✓ Geostrophic wind blows parallel to the isobars, with the pressure gradient force balanced by the Coriolis force.
⚠ Thinking the Coriolis Force is Strongest at the Equator
✗ The Coriolis force is maximum at the equator.
→ The Coriolis force is directly proportional to the angle of latitude; it is maximum at the poles and absent (zero) at the equator.
✓ The Coriolis force is maximum at the poles and absent at the equator.
⚠ Confusing the ITCZ with the Subtropical High
✗ The Inter Tropical Convergence Zone is a high-pressure belt.
→ The ITCZ is a low-pressure zone where winds converge and air rises, not a high-pressure belt.
✓ The Inter Tropical Convergence Zone (ITCZ) is a low-pressure zone near the equator where trade winds converge and air rises.
⚠ Assuming Tropical Cyclones Have Frontal Systems
✗ Tropical cyclones have warm and cold fronts like extra tropical cyclones.
→ Tropical cyclones do not have frontal systems; they are warm-core systems. Extra tropical cyclones have clear frontal systems.
✓ Tropical cyclones do not have frontal systems; they are characterised by an eye and an eyewall, unlike extra tropical cyclones which have warm and cold fronts.
⚠ Misstating the Direction of Movement of Cyclones
✗ Extra tropical cyclones move from east to west.
→ Extra tropical cyclones move from west to east, while tropical cyclones move from east to west.
✓ Extra tropical cyclones move from west to east, whereas tropical cyclones move from east to west.
Compare & Contrast
Wind circulation in the Northern Hemisphere
Cyclone (Low Pressure)
Anticyclone (High Pressure)
In a cyclone, wind circulates anticlockwise; in an anticyclone, wind circulates clockwise.
Presence of frontal systems
Extra Tropical Cyclone
Tropical Cyclone
Extra tropical cyclones have a clear frontal system (warm and cold fronts); tropical cyclones do not have frontal systems.
Time of occurrence and direction
Sea Breeze
Land Breeze
Sea breeze blows from sea to land during the day; land breeze blows from land to sea at night.
Latitudinal location and direction of surface winds
Hadley Cell
Ferrel Cell
The Hadley Cell operates in the tropics (0-30°) with surface easterlies; the Ferrel Cell operates in the middle latitudes (30-60°) with surface westerlies.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Short Answer (3 marks), 1 Long Answer (5 marks) — total 10 marks typically.
Most asked: Factors affecting wind speed and direction (pressure gradient force, Coriolis force, friction) · General circulation of the atmosphere (three-cell model) · Formation and characteristics of tropical cyclones · Differences between extra tropical and tropical cyclones · Pressure belts and their seasonal migration · El Nino and ENSO
Don't skip: Long answer question on factors affecting wind speed and direction (5 marks); Long answer question on the formation and structure of tropical cyclones (5 marks); Diagram-based question on general circulation of the atmosphere (3-5 marks)
One-Line Revision (read aloud before the exam)
  1. Atmospheric pressure is the weight of a column of air per unit area from sea level to the top of the atmosphere, measured in millibars.
  2. Pressure decreases with height at about 1 mb per 10 m in the lower atmosphere.
  3. Isobars are lines joining places of equal pressure on a weather map.
  4. The pressure gradient force is the rate of change of pressure with distance; it is strong where isobars are close.
  5. The Coriolis force deflects wind to the right in the northern hemisphere and to the left in the southern hemisphere, and is zero at the equator.
  6. Geostrophic wind blows parallel to isobars when the pressure gradient force is balanced by the Coriolis force above the friction layer.
  7. In the northern hemisphere, cyclonic circulation is anticlockwise and anticyclonic circulation is clockwise.
  8. The ITCZ is a low-pressure zone near the equator where trade winds converge and air rises.
  9. The three cells of general circulation are the Hadley Cell, Ferrel Cell, and Polar Cell.
  10. Air masses are large bodies of air with uniform temperature and humidity, classified as mT, cT, mP, cP, and cA.
  11. A front is the boundary between two different air masses; types include cold, warm, stationary, and occluded.
  12. Extra tropical cyclones form along the polar front and have clear frontal systems; they move from west to east.
  13. Tropical cyclones originate over warm oceans (sea surface temp >27°C) and have an eye and eyewall; they move from east to west.
  14. El Nino is the appearance of warm water off the coast of Peru, associated with the Southern Oscillation (ENSO).
  15. Sea breeze blows from sea to land during the day; land breeze blows from land to sea at night.
  16. Valley breeze is an upslope wind during the day; mountain wind is a downslope wind at night.
  17. Thunderstorms are caused by intense convection on moist hot days, producing cumulonimbus clouds and lightning.
  18. A tornado is a violent spiralling wind descending from a thunderstorm, with very low pressure at the centre.
  19. The vertical pressure gradient force is balanced by gravity, so we do not experience strong upward winds.
  20. The pressure belts (equatorial low, subtropical highs, subpolar lows, polar highs) migrate with the apparent movement of the sun.

MCQs — 10 Questions with Answers

Q1. At sea level, the average atmospheric pressure is approximately:

  • A. 1,013.25 millibars ✓
  • B. 500 millibars
  • C. 750 millibars
  • D. 2,000 millibars

Answer: A — The standard atmospheric pressure at sea level is 1,013.25 mb; all other options are incorrect standard values.

Q2. When isobars are closely spaced on a weather map, it indicates:

  • A. Low wind velocity and weak pressure gradient
  • B. High wind velocity and strong pressure gradient ✓
  • C. No relationship between wind and pressure
  • D. Constant wind speed regardless of isobar spacing

Answer: B — Close isobars mean pressure changes rapidly over a short distance, creating a steep pressure gradient and causing stronger wind.

Q3. The Coriolis force is:

  • A. Maximum at the equator and zero at the poles
  • B. Zero at the equator and maximum at the poles ✓
  • C. Constant everywhere on Earth
  • D. Directly proportional to temperature

Answer: B — The Coriolis force depends on latitude; it is zero at the equator (no deflection) and maximum at the poles.

Q4. In the Northern Hemisphere, winds in a cyclone (low-pressure system) move:

  • A. Clockwise around the centre
  • B. Anticlockwise around the centre ✓
  • C. Straight toward the centre
  • D. Straight away from the centre

Answer: B — In the Northern Hemisphere, low-pressure systems (cyclones) have anticlockwise wind circulation due to the combined effect of pressure gradient and Coriolis force.

Q5. Tropical cyclones do not form near the equator because:

  • A. Temperature is too high
  • B. Pressure is always high near the equator
  • C. The Coriolis force is zero, so low pressure fills instead of intensifying ✓
  • D. Humidity is too low near the equator

Answer: C — At the equator, Coriolis force = 0, so wind blows perpendicular to isobars and fills low-pressure areas instead of creating the rotating circulation needed for cyclone formation.

Q6. The wind that flows parallel to isobars in the upper atmosphere where pressure gradient and Coriolis forces are balanced is called:

  • A. Trade wind
  • B. Jet stream
  • C. Geostrophic wind ✓
  • D. Monsoon wind

Answer: C — Geostrophic wind is the upper-atmosphere wind (2–3 km height) where the pressure gradient force is balanced by the Coriolis force, causing wind to blow parallel to isobars.

Q7. Which statement is NOT correct regarding frictional force?

  • A. Friction is greatest at the Earth's surface
  • B. Friction extends up to 1–3 km elevation
  • C. Friction is maximum over ocean surfaces ✓
  • D. Friction reduces wind speed near the surface

Answer: C — Friction is minimal over ocean surfaces because water offers less resistance; friction is greatest over rough land surfaces.

Q8. Study the pressure distribution table: At 5 km elevation, the pressure is 540.48 mb. What is the approximate pressure decrease from sea level to 5 km?

  • A. 473 mb ✓
  • B. 540 mb
  • C. 1,013 mb
  • D. 898 mb

Answer: A — Pressure decrease = 1,013.25 mb (sea level) − 540.48 mb (5 km) = 472.77 mb ≈ 473 mb.

Q9. Which of the following statements about pressure belts is correct? (I) Equatorial low oscillates with the apparent movement of the sun. (II) Subtropical highs at 30° N and 30° S are permanent features.

  • A. Both I and II are correct ✓
  • B. Only I is correct
  • C. Only II is correct
  • D. Neither I nor II is correct

Answer: A — Both statements are correct: equatorial low pressure shifts seasonally with the sun's apparent movement, and subtropical highs at 30° N/S are permanent pressure cells.

Q10. If the pressure gradient force acts perpendicular to an isobar and the Coriolis force acts perpendicular to the pressure gradient force, explain why wind in the Northern Hemisphere spirals anticlockwise around a low-pressure system rather than blowing straight toward the centre.

  • A. Wind is deflected by Coriolis to the right, which redirects it to spiral anticlockwise around the low ✓
  • B. Wind is deflected by Coriolis to the left, which combined with pressure gradient force creates anticlockwise spiral
  • C. Wind is attracted to the cold air in low-pressure systems
  • D. Friction at the surface removes the northward component of wind

Answer: A — In the Northern Hemisphere, the Coriolis force deflects wind to the right, and as the wind curves, this perpendicular deflection causes it to spiral anticlockwise around the low-pressure centre.

Flashcards

What is atmospheric pressure and in what units is it measured?

Atmospheric pressure is the weight of a column of air from sea level to the top of the atmosphere per unit area, measured in millibars (mb); average at sea level is 1,013.25 mb.

Why does pressure decrease with altitude?

Air is denser at the surface due to gravity pulling air downward, so it compresses more near sea level; as you go higher, there is less air above to create weight pressure.

What is an isobar and what does its spacing tell you?

An isobar is a line connecting places with equal atmospheric pressure; when isobars are close together, the pressure gradient is steep and wind speed is high, and vice versa.

Define the Coriolis force and state where it is zero.

The Coriolis force is the deflection of wind caused by Earth's rotation, deflecting it right in the Northern Hemisphere and left in the Southern Hemisphere; it is zero at the equator and maximum at the poles.

What is the difference between a cyclone and an anticyclone in terms of wind direction?

In a cyclone (low pressure), winds spiral anticlockwise in the Northern Hemisphere; in an anticyclone (high pressure), winds spiral clockwise in the Northern Hemisphere.

Explain why tropical cyclones do not form near the equator.

At the equator, the Coriolis force is zero, so wind blows perpendicular to isobars and fills low-pressure areas instead of intensifying them into rotating storm systems.

What is a geostrophic wind?

A geostrophic wind is the wind in the upper atmosphere (2–3 km above surface) where pressure gradient force and Coriolis force are balanced, causing wind to blow parallel to isobars.

State the three forces that control surface wind and which is the most effective in driving wind initiation.

The three forces are pressure gradient force, Coriolis force, and frictional force; the pressure gradient force is the primary driver that initiates wind motion.

Why do pressure belts shift seasonally and in which direction?

Pressure belts shift seasonally because they follow the apparent movement of the sun; in the Northern Hemisphere winter they move southwards, and in summer they move northwards.

How does friction affect wind at the surface compared to the upper atmosphere?

Friction is greatest at the surface and reduces wind speed; its influence extends up to 1–3 km height, whereas upper atmosphere winds are free from frictional effects and blow parallel to isobars.

Important Board Questions

Define atmospheric pressure and state its average value at sea level. [2 marks]

Define as weight of air column per unit area; state standard sea level value in millibars from the chapter.

Explain with diagram how the three forces (pressure gradient, Coriolis, and frictional) control the velocity and direction of surface wind. Why does wind in the upper atmosphere blow parallel to isobars? [5 marks]

Draw or describe direction of three forces; explain how pressure gradient initiates wind and Coriolis deflects it; state that at 2–3 km height, friction is absent and balanced PGF–Coriolis causes geostrophic wind parallel to isobars.

Analyse the global distribution of pressure belts shown in Figures 9.2 and 9.3 (January and July). Explain why equatorial low and subtropical highs shift seasonally, and why tropical cyclones cannot form at the equator despite the presence of low pressure. Support your answer with reference to the Coriolis force. [6 marks]

Explain pressure belts follow sun's apparent movement (ITCZ shifts); describe seasonal shift of subtropical highs northward in July and southward in January; state Coriolis = 0 at equator so wind blows perpendicular to isobars and fills low pressure instead of intensifying cyclone structure.

Assertion-Reason Questions for Atmospheric Pressure and Winds

Q1. Assertion (A):

The equatorial low-pressure belt cannot sustain tropical cyclones due to physical constraints.

Reason (R):

The Coriolis force is zero at the equator, preventing the rotating circulation pattern necessary for cyclone formation.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why cyclones do not form near the equator. The absence of Coriolis force at the equator means low-pressure systems cannot organize into rotating storm systems and instead get filled.

Q2. Assertion (A):

Atmospheric pressure decreases with height at a constant rate throughout the troposphere.

Reason (R):

The density of air decreases with altitude, causing the rate of pressure decrease to vary non-linearly.

Answer: D — A is false but R is true — The assertion is false because pressure does not decrease at a constant rate—it decreases more rapidly near the surface and more slowly aloft. The reason is true but explains why the assertion is false. The relationship is non-linear.

Q3. Assertion (A):

Wind blows perpendicular to isobars at the Earth's surface due to pressure gradient force alone.

Reason (R):

Frictional force at the surface causes wind to flow at an angle to isobars rather than perpendicular to them.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true. Wind at the surface does blow at angles to isobars (not perpendicular), and friction is responsible for this deviation from the theoretical perpendicular flow that would occur if only pressure gradient and Coriolis forces were present. However, the reason describes a consequence rather than explaining why the assertion is true—the Coriolis force causes directional deflection, while friction modifies this further.

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Case-Based Questions (CBQ) for Atmospheric Pressure and Winds

Case Study:

A meteorology team is tracking a monsoon depression over the Bay of Bengal in July. Pressure measurements from multiple weather stations show: (1) Central pressure: 990 mb; (2) Surrounding areas at 50 km distance: 1005 mb; (3) At 100 km distance: 1012 mb. The team observes strong convergent wind patterns and rising air motion near the center. They also note that tropical storms in this region rotate clockwise when viewed from above, indicating the Southern Hemisphere characteristics affecting the circulation pattern.

Q1. What can be inferred about the pressure gradient strength in this weather system?

  • A. The pressure gradient is weak because the pressure difference is only 22 mb over 100 km
  • B. The pressure gradient is strong because isobars are closely spaced over a short distance (100 km shows 22 mb change) ✓
  • C. The pressure gradient cannot be determined without temperature data
  • D. The pressure gradient is uniform throughout the region

Answer: B

Q2. Why is the Coriolis force critical for the rotation pattern observed in this monsoon depression?

  • A. The Coriolis force directly creates the pressure low; without it, no depression would form
  • B. The Coriolis force, being maximum at the Bay of Bengal's latitude (~15°S in context), deflects converging wind into clockwise rotation ✓
  • C. The Coriolis force is zero near the equatorial Bay of Bengal, allowing clockwise rotation freely
  • D. Rotation pattern is independent of Coriolis force and depends only on surface friction

Answer: B

Q3. Given the pressure values, approximately what is the pressure gradient rate between the center and 100 km distance?

  • A. 2.2 mb per 10 km ✓
  • B. 0.22 mb per 10 km
  • C. 22 mb per 10 km
  • D. 0.022 mb per 10 km

Answer: A

Q4. How does the rising air motion at the center of this depression relate to the pressure system and subsequent weather?

  • A. Rising air is unrelated to low pressure; it occurs due to heating alone
  • B. Rising air indicates the system will weaken because air is escaping upward
  • C. Rising air over low-pressure areas leads to cooling, condensation, cloud formation, and precipitation, typical of monsoon depressions ✓
  • D. Rising air only occurs at the surface and does not extend to upper atmosphere

Answer: C

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