The irregularity in the amount of rainfall in different parts of north Indian plains is mainly due to :

1. Core Mechanism of the Indian Monsoon (basic)

To master the Indian Monsoon, we must first view it as a massive, seasonal thermal engine. At its most basic level, the monsoon is driven by differential heating: the fact that land heats up and cools down much faster than water. During the scorching summer months, the Indian landmass becomes significantly hotter than the surrounding Indian Ocean. This intense heat causes air over the land to rise, creating a powerful Low Pressure (LP) zone, while the relatively cooler ocean remains a High Pressure (HP) zone INDIA PHYSICAL ENVIRONMENT, Geography Class XI, Chapter: Climate, p.29. Because winds naturally flow from high to low pressure, moisture-heavy air from the sea rushes toward the subcontinent, bringing the life-giving rains Certificate Physical and Human Geography, GC Leong, Chapter: Climate, p.141.

However, the monsoon isn't just one long, steady rainstorm; it is famously irregular. This "patchiness" of rainfall is largely governed by the Monsoon Trough. This is an elongated area of low pressure that typically stretches from Northwest India down to the Bay of Bengal. Think of this trough as the "steering wheel" of the monsoon. The axis of this trough is not fixed—it oscillates (moves) north and south based on meteorological conditions NCERT Class IX, Contemporary India-I, Chapter 4: Climate, p.30.

Position of Trough Axis	Impact on Rainfall
Over the North Indian Plains	Heavy rainfall across the plains of North India.
Shifted North toward Himalayas	'Breaks' (dry spells) in the plains; heavy rain in mountain catchments.

While the thermal concept (first proposed by Halley) explains the basic wind reversal, modern meteorology highlights that the monsoon is a complex amalgamation of convectional, mountain-influenced (orographic), and cyclonic rainfall Geography of India, Majid Husain, Climate of India, p.3. For instance, the frequency and path of tropical depressions (small low-pressure systems) that follow the monsoon trough are what ultimately decide which specific districts get flooded and which remain dry in a given week NCERT Class IX, Contemporary India-I, Chapter 4: Climate, p.32.

Sources: INDIA PHYSICAL ENVIRONMENT, Geography Class XI, Climate, p.29; Certificate Physical and Human Geography, GC Leong, Climate, p.141; NCERT Class IX, Contemporary India-I, Chapter 4: Climate, p.30-32; Geography of India, Majid Husain, Climate of India, p.3

2. Branches of the Advancing Monsoon (basic)

When the South-West Monsoon winds approach the Indian landmass, they don't arrive as a single uniform wall of air. Instead, the tapered shape of the Indian peninsula acts like a giant wedge, splitting the moisture-laden winds into two distinct streams: the Arabian Sea branch and the Bay of Bengal branch INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.35. These winds originate as South-East trade winds in the southern hemisphere, crossing the equator and turning right due to the Coriolis force to become the South-West monsoon CONTEMPORARY INDIA-I, Geography, Class IX . NCERT(Revised ed 2025), Climate, p.30.

While both branches carry immense moisture, they behave differently due to the geography they encounter. The Arabian Sea branch is technically more powerful because the Arabian Sea is much larger than the Bay of Bengal. However, the Bay of Bengal branch is unique because of how it is diverted. As it moves northwards, it strikes the coast of Myanmar and Bangladesh. It is the Arakan Hills along the Myanmar coast that deflect a significant portion of this branch westward, forcing it to enter India from the south and southeast rather than the southwest INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.37.

To understand the characteristics of these two "water carriers," we can look at their oceanic environments:

Feature	Arabian Sea Branch	Bay of Bengal Branch
Size	Much larger surface area (approx. 3.8 million sq km).	Smaller surface area (approx. 2.1 million sq km).
Temperature	Relatively cooler, partly due to the Somali Current.	Relatively warmer, which aids evaporation and cyclone formation.
Salinity	Higher salinity due to high evaporation and less river runoff.	Lower salinity due to massive freshwater influx from the Ganga and Brahmaputra.

Geography of India, Majid Husain, Climate of India, p.28; Physical Geography by PMF IAS, Tropical Cyclones, p.358

Once the Bay of Bengal branch enters India, the mighty Himalayas and the thermal low pressure in Northwest India act as guides. This branch splits again: one part moves up the Brahmaputra valley, causing heavy rains in the Northeast, while the other travels westward along the Ganga plains, eventually meeting the Arabian Sea branch in the Punjab plains INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.37.

Sources: INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.35, 37; CONTEMPORARY INDIA-I, Geography, Class IX (NCERT 2025 ed.), Climate, p.30; Geography of India, Majid Husain, Climate of India, p.28; Physical Geography by PMF IAS, Tropical Cyclones, p.358

3. Role of Relief and Orography (intermediate)

In the study of the Indian Monsoon, Relief (Orography) is perhaps the most decisive factor in determining where and how much it rains. While thermal differences (land vs. sea) initiate the winds, the physical geography of India acts as a series of barriers and channels that dictate the final distribution of moisture. Without these mountain ranges, the monsoon winds would simply blow across the subcontinent without shedding their moisture, potentially leaving northern India as a vast desert Majid Husain, Geography of India, Physiography, p.28.

The Himalayas play a dual role. First, they act as an impenetrable wall that intercepts the moisture-laden summer monsoon winds from both the Arabian Sea and the Bay of Bengal, forcing them to rise and release precipitation. Second, they perform a channelling effect: when the Bay of Bengal branch hits the mountains in the northeast, it is deflected westward along the Ganga Plain Majid Husain, Geography of India, Climate of India, p.17. Similarly, the Western Ghats stand perpendicular to the Arabian Sea branch, causing heavy orographic rainfall on the windward side (coastal plains), while leaving the interior Deccan Plateau in a rain shadow with significantly less moisture NCERT Class XI, India Physical Environment, Climate, p.38.

The spatial irregularity of rainfall is also governed by the interaction between relief and the Monsoon Trough—an elongated low-pressure zone. The axis of this trough is not fixed; it oscillates. When the axis shifts toward the Himalayan foothills, the mountain catchments receive intense rainfall (often leading to floods in the plains), while the plains themselves experience a "break" or dry spell NCERT Class IX, Contemporary India-I, Climate, p.32. In the Northeast, the unique funnel-like shape of the Garo, Khasi, and Jaintia hills traps the monsoon winds, leading to world-record precipitation levels in places like Mawsynram Majid Husain, Geography of India, Climate of India, p.30.

Relief Feature	Impact on Monsoon	Resulting Rainfall Pattern
Himalayas	Barrier and Deflector	Heavy rain in sub-Himalayan belt; channelling moisture to North Indian plains.
Western Ghats	Orographic Obstruction	High rainfall on the West Coast (>250cm); semi-arid rain shadow in the interior.
Meghalaya Hills	Funneling Effect	Extreme rainfall (>1000cm) at Cherrapunji and Mawsynram.

Sources: Geography of India, Majid Husain, Physiography, p.28; Geography of India, Majid Husain, Climate of India, p.17, 30; NCERT Class XI, India Physical Environment, Climate, p.38; NCERT Class IX, Contemporary India-I, Climate, p.32

4. Tropical Depressions and Cyclonic Activity (intermediate)

Hello! It is wonderful to have you back as we continue our journey into the Indian Monsoon. Having understood the broad mechanics of wind shifts, we now look at the 'delivery vehicles' of monsoon rain: Tropical Depressions and the Monsoon Trough. While the Southwest Monsoon winds bring moisture, the actual distribution of rain depends on the behavior of an elongated zone of low pressure known as the Monsoon Trough. This trough extends from Northwest India to the head of the Bay of Bengal and serves as the 'track' that tropical depressions follow as they move inland NCERT Class IX, Climate, p. 30.

The Monsoon Trough is not static; it is highly dynamic and oscillates northward and southward. This movement is the primary reason for the spatial irregularity and the famous 'breaks' in monsoon rainfall. When the axis of the trough is over the North Indian plains, rainfall is heavy and widespread in those regions. However, when the axis shifts north towards the Himalayan foothills, the plains go dry, and the mountain catchment areas receive intense precipitation, often leading to devastating floods in the rivers downstream NCERT Class IX, Climate, p. 32.

While the trough provides the path, Tropical Depressions provide the intensity. These are low-pressure systems that primarily form at the head of the Bay of Bengal. Interestingly, during the peak monsoon months, these systems rarely intensify into full-blown 'Super Cyclones.' This is because the Bay of Bengal is narrower in the north; the depressions make landfall within a day or two, giving them a shorter oceanic stay and preventing the massive accumulation of energy required for higher-intensity cyclones PMF IAS, Tropical Cyclones, p. 360. On average, the Bay of Bengal sees significantly more of these systems than the Arabian Sea, maintaining a ratio of roughly 4:1 PMF IAS, Tropical Cyclones, p. 357.

The following table summarizes how the position of the Trough impacts the weather across India:

Trough Position	Rainfall in North Indian Plains	Rainfall in Himalayan Foothills
Axis over the Plains	Abundant/Heavy	Normal to Low
Axis at Himalayan Foothills	'Break' (Dry Spell)	Extremely Heavy/Floods

Sources: CONTEMPORARY INDIA-I, Geography, Class IX. NCERT (Revised ed 2025), Chapter 4: Climate, p.30, 32; Physical Geography by PMF IAS, Tropical Cyclones, p.357, 360

5. Global Climatic Teleconnections (ENSO & IOD) (intermediate)

While the Indian monsoon is a local phenomenon, it is deeply influenced by teleconnections—climatic links between distant parts of the globe. The most significant of these is the ENSO (El Niño Southern Oscillation). In a normal year, warm water and low pressure are concentrated over the Western Pacific (near Indonesia), which aids the rising limb of air that eventually fuels the Indian monsoon. However, during an El Niño year, this pool of warm water shifts eastward toward the coast of South America Geography of India, Majid Husain, Climate of India, p.10. This eastward shift causes high pressure over the Western Pacific and Indonesia, suppressing the convection needed for a strong Indian monsoon and often leading to droughts Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. Conversely, La Niña (the cold phase) typically enhances monsoon rainfall.

Closer to home, the Indian Ocean Dipole (IOD) serves as the monsoon's internal regulator. The IOD is defined by the difference in sea surface temperatures between the western Indian Ocean (Arabian Sea) and the eastern Indian Ocean (south of Indonesia) Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415. We categorize it into two primary phases:

Feature	Positive IOD (+IOD)	Negative IOD (-IOD)
Temperature	Western Indian Ocean is warmer than usual.	Eastern Indian Ocean (near Indonesia) is warmer.
Winds	Blow from East to West (toward Africa/India).	Blow from West to East (toward Indonesia).
Monsoon Impact	Beneficial; increases rainfall in India.	Negative; decreases rainfall in India.

The beauty of these systems lies in their interaction. A Positive IOD can act as a "shield," negating the drying effects of an El Niño. For instance, in 1997, despite a massive El Niño that should have caused a drought, India received normal rainfall because a powerful Positive IOD compensated for the Pacific deficit Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.416. Just as ENSO has an atmospheric component (the Southern Oscillation), the IOD has an atmospheric partner called EQUINOO (Equatorial Indian Ocean Oscillation), which involves the shifting of pressure cells between the Bay of Bengal and the Arabian Sea Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415.

Sources: Geography of India, Climate of India, p.10; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.415; Physical Geography by PMF IAS, El Nino, La Nina & El Nino Modoki, p.416; Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.54

6. The Monsoon Trough: Anatomy and Movement (exam-level)

To understand the spatial and temporal distribution of rainfall in India, we must look at the Monsoon Trough. At its core, this trough is simply the Inter Tropical Convergence Zone (ITCZ) that has migrated northward during the summer due to the intense heating of the Indian subcontinent. In July, this elongated low-pressure zone typically settles between 20°N and 25°N latitudes, stretching from the Thar Desert in the northwest to the Chotanagpur Plateau in the east-southeast INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.30, 34. It acts like a massive atmospheric magnet, pulling in moisture-laden winds from the surrounding seas.

The most fascinating aspect of the monsoon trough is its oscillation. It is not a stationary feature; its axis constantly shifts northward and southward. This movement is the primary reason for the "wet" and "dry" spells of the monsoon. When the axis of the trough sits firmly over the Northern Plains, the region receives widespread and consistent rainfall. However, if the axis shifts further north toward the Himalayan foothills, a unique phenomenon occurs: the plains experience a "break" or a dry spell, while the mountainous catchment areas of Himalayan rivers receive heavy downpours, often leading to sudden floods CONTEMPORARY INDIA-I, Geography, Class IX (NCERT 2025 ed.), Climate, p.30.

Furthermore, the trough serves as a highway for tropical depressions. These low-pressure systems typically form at the head of the Bay of Bengal and travel inland along the axis of the monsoon trough. The intensity and the specific path these depressions take—determined by the trough's position—ultimately decide which parts of India get surplus rain and which face a deficit in a given year Physical Geography by PMF IAS, Tropical Cyclones, p.360. In years when the eastern end of the trough stays further south (near Odisha or Chhattisgarh), the rainfall patterns shift accordingly, sometimes even allowing a separate low-pressure system to develop over the Assam plains Geography of India, Majid Husain, Climate of India, p.15.

Position of Trough Axis	Impact on North Indian Plains	Impact on Himalayan Region
Over the Gangetic Plains	Active Monsoon (Abundant Rain)	Normal precipitation
Shifted to Himalayan Foothills	"Break" in Monsoon (Dry Spells)	Heavy rain and potential floods

Sources: INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.30, 34; CONTEMPORARY INDIA-I, Geography, Class IX (NCERT 2025 ed.), Climate, p.30; Physical Geography by PMF IAS, Tropical Cyclones, p.360; Geography of India, Majid Husain, Climate of India, p.15

7. Breaks in the Monsoon (exam-level)

The Indian monsoon is not a continuous, non-stop downpour from June to September. Instead, it is pulsatory in nature, characterized by wet spells interspersed with dry intervals. When the rain fails to occur for one or more weeks during the peak rainy season, we call this a 'Break' in the Monsoon INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.31. These breaks are a natural part of the monsoon's rhythm, but their duration and frequency can mean the difference between a bumper harvest and a regional drought.

The most critical factor governing these breaks in North India is the movement of the Monsoon Trough (the axis of low pressure). Think of this trough as the "magnet" for rain-bearing clouds. Under normal conditions, this axis sits over the Indo-Gangetic plains, resulting in widespread rainfall. However, when this axis shifts northward toward the foothills of the Himalayas, the plains are left dry, while the mountainous catchment areas receive heavy precipitation, often leading to devastating floods in the plains downstream CONTEMPORARY INDIA-I, Geography, Class IX (NCERT 2025 ed.), Climate, p.30.

It is important to note that the causes of breaks vary by geography:

• North Indian Plains: Caused by the northward migration of the monsoon trough or a decrease in the frequency of tropical depressions from the Bay of Bengal INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.31.
• West Coast: Occurs when the moisture-laden winds blow parallel to the coast rather than hitting the Western Ghats perpendicularly.

These breaks usually last for 3 to 5 days but can sometimes extend for several weeks. When a break persists for a long duration (10–20 days or more), it significantly disrupts the agricultural cycle Geography of India, Majid Husain, Climate of India, p.15.

Feature	Active Monsoon Phase	Break Monsoon Phase
Trough Position	Over the North Indian Plains	Shifted to Himalayan Foothills
Rainfall in Plains	Abundant and widespread	Scanty or dry spells
Himalayan Rivers	Normal flow	Heavy rain in catchments; potential floods

Sources: INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.31; CONTEMPORARY INDIA-I, Geography, Class IX (NCERT 2025 ed.), Climate, p.30; Geography of India, Majid Husain, Climate of India, p.15

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental mechanics of the Indian Monsoon, this question tests your ability to apply the concept of the Monsoon Trough. The North Indian plains do not receive uniform rainfall because the monsoon is not a static event; it is governed by a dynamic axis of low pressure that stretches from the Thar Desert to the Bay of Bengal. As you learned in CONTEMPORARY INDIA-I, Geography, Class IX . NCERT (Revised ed 2025), this trough is the "steering wheel" of the monsoon. When the axis sits over the plains, rainfall is heavy, but its spatial migration—moving north toward the Himalayas or south toward the Peninsula—is what creates the specific patterns of wet and dry spells across the region.

To arrive at the correct answer, (B) variation in the location of the axis of low pressure trough, you must focus on the word "irregularity." While the heat low in the northwest (Option A) acts as the initial vacuum that pulls the moisture-laden winds into India, it doesn't explain why rainfall varies across different parts of the plains once the winds have arrived. The trough's oscillation is the primary mechanism: when the axis shifts to the Himalayan foothills, the plains experience a "break" in the monsoon, shifting the heavy rain to the mountain catchment areas. This movement is the direct cause of the spatial and temporal irregularity you see in the data.

UPSC often uses "partially correct" distractors to test your precision. Option A is a classic trap; it describes the trigger for the monsoon's arrival, not the distribution of its rain. Option C mentions cyclones, which do bring rain, but their frequency and path are actually determined by the position of the trough itself, making the trough the more fundamental cause. Finally, Option D is too broad—while moisture levels change yearly, it is the trough’s migration that dictates where that moisture actually falls as rain within a single season. By identifying the trough's axis as the primary spatial regulator, you avoid these common traps.