Seasons of India

The India Meteorological Department (IMD) officially recognizes four distinct seasons in its annual climatic cycle. The sequence, timing, and characteristics of these seasons are fundamentally driven by the apparent migration of the Sun, the shifting of the Inter-Tropical Convergence Zone (ITCZ), and the seasonal reversal of pressure gradients across the Indian subcontinent and the Indian Ocean.

The Cold Weather Season (Winter)

Spatial and Temporal Distribution

• Duration: This season starts in mid-November and extends until February across northern India. December and January are the coldest months.
• Temperature Gradient: Clear skies, low humidity, and a steep temperature gradient from south to north characterize this period. While peninsular India maintains warm conditions (average temperatures around 24°C to 25°C), northern plains experience sharp drops, with average temperatures hovering between 10°C and 15°C.

Pressure and Wind Mechanics

• High-Pressure Build-up: Intense cooling over Central Asia and the Tibetan Plateau creates a strong high-pressure center. A secondary high-pressure cell develops over the northwestern Indian plains (around the Indus-Ganga valley).
• Northeast Monsoon: Winds blow outward from this continental high-pressure zone toward the equatorial low-pressure oceanic regions. These are the Northeast Trade Winds. Being continental in origin, they are mostly dry, bringing clear skies and crisp weather to most of India.

Precipitation and Associated Phenomena

• Western Disturbances: Shallow, non-tropical cyclonic storms originate over the Mediterranean Sea and travel eastward under the influence of the Subtropical Westerly Jet Stream. They enter India through Pakistan and Iran, bringing crucial winter rainfall to the plains of Punjab, Haryana, and western Uttar Pradesh, and heavy snowfall to the Jammu & Kashmir, Ladakh, Himachal Pradesh, and Uttarakhand Himalayas.
• Winter Monsoon Rainfall: As the Northeast Monsoon winds travel across the Bay of Bengal, they pick up moisture and strike the Coromandel Coast. This causes significant winter rainfall in coastal Tamil Nadu and parts of southern Andhra Pradesh between October and December.
• Frost and Fog: Severe frost is common in the elevated plains of Punjab and Rajasthan. Dense radiation fog develops over the Indo-Gangetic plains due to high relative humidity, calm winds, and nocturnal cooling.

The Hot Weather Season (Pre-Monsoon / Summer)

Spatial and Temporal Distribution

• Duration: March to May marks the period of continuous heating and rising temperatures across the Indian landmass.
• Northward Shifting Heat Belt: Due to the apparent northward movement of the sun, the global heat belt shifts north. In March, the highest temperatures (around 38°C) occur on the Deccan Plateau. By April, heat zones move into Madhya Pradesh and Gujarat (42°C), and by May, northwestern India records extreme temperatures reaching 45°C to 48°C.

Pressure and Atmospheric Circulation

• Elongated Low-Pressure Trough: Continuous thermal heating results in the formation of an elongated low-pressure area extending from the Thar Desert in the northwest to Chota Nagpur Plateau in the east. This is called the Monsoon Trough.
• The ‘Loo’ Wind: Strong, dusty, scorching, and hyper-dry winds blow during the afternoon hours over northern and northwestern India. Direct exposure to these winds can cause fatal heatstrokes.

Localized Thunderstorms and Pre-Monsoon Showers

The convergence of dry continental air masses with humid maritime air masses around the low-pressure trough triggers violent local storms characterized by torrential downpours, hail, and fierce squalls. These storms are known by distinct regional names based on their economic impacts:

The Southwest Monsoon Season (Advancing Monsoon / Rainy Season)

Spatial and Temporal Distribution

• Duration: June to September is the primary rainy season, providing over 75% of India’s annual precipitation.
• The “Burst” of Monsoon: By early June, the low-pressure conditions over northern plains intensify enough to rapidly attract the Southeast Trade Winds from the Southern Hemisphere. As these winds cross the equator, the Coriolis force deflects them into Southwest Monsoon winds. Their arrival is marked by a sudden, violent onset of heavy rainfall accompanied by thunder and lightning, termed the “Burst of the Monsoon”.

Branches of the Advancing Monsoon

The peninsular shape of India splits the approaching Southwest Monsoon into two distinct operational branches:

• The Arabian Sea Branch: This branch approaches India from the west and splits further into three distinct streams:
  1. Western Ghats Stream: Obstructed by the Western Ghats, it rises abruptly and unloads heavy orographic rainfall exceeding 250 cm on the windward side. As it crosses to the leeward Deccan Plateau, it creates a prominent rain-shadow zone.
  2. Narmada-Tapi Trough Stream: Enters central India through the Narmada and Tapi valleys, causing substantial rainfall over Madhya Pradesh without meeting significant mountain barriers until the Chota Nagpur Plateau.
  3. Saurashtra-Aravali Stream: Passes over Gujarat and runs completely parallel to the Aravali Range in Rajasthan. Because the mountains do not obstruct the winds, this branch causes negligible rainfall in Rajasthan, leading to arid conditions.
• The Bay of Bengal Branch: This branch moves across the Bay of Bengal toward northeastern India and the Indo-Gangetic plains:
  1. Northeast Stream: Moves toward the Arakan Hills in Myanmar and Northeast India. It gets trapped in the funnel-shaped Khasi Hills, where Mawsynram and Cherrapunji record the highest average annual rainfall on Earth.
  2. Ganga Plains Stream: Deflected by the Himalayan barrier, this stream moves westward up the Indo-Gangetic plains. The volume of rainfall decreases progressively from east to west (e.g., Kolkata receives more rain than Patna, which receives more than Delhi).

Breaks in the Monsoon

During July and August, there are distinct periods when the monsoon trough shifts closer to the Himalayan foothills, causing rainfall to cease entirely over the northern plains while heavy downpours trigger landslides in the mountains. This dry spell within the rainy season is called a “Break in the Monsoon”.

The Retreating Monsoon Season (Cooling Transition)

Spatial and Temporal Distribution

• Duration: October and November mark the transition from the wet rainy season to dry winter conditions.
• Southward Retreat: As the sun moves south, the low-pressure monsoon trough over northern plains weakens and begins migrating southward. By early October, the monsoon retreats from northern India, and by December, it clears the entire subcontinent.

Climatic Characteristics and “October Heat”

• Atmospheric State: The retreat is marked by clear, cloudless skies and an increase in surface temperatures over land that is still saturated with moisture.
• October Heat: The combination of high daytime temperatures and oppressive relative humidity creates stifling, uncomfortable weather conditions known across India as “October Heat”.

Cyclonogenesis and Coastal Inundation

• Tropical Cyclones: The retreating low-pressure center shifts into the Bay of Bengal during October and November. This thermal shift triggers severe cyclonogenesis (the formation and intensification of tropical cyclones).
• Coastal Impact: These destructive cyclones pick up energy from the warm waters of the Bay of Bengal and move westward, striking the densely populated deltas of the Mahanadi, Godavari, Krishna, and Cauvery rivers along the Odisha, Andhra Pradesh, and Tamil Nadu coasts, causing heavy rainfall and storm surges.

UPSC Prelims Core Facts and Climatic Trivia

• Mawsynram vs. Cherrapunji: Mawsynram, located on the crest of the Southern Range of the Khasi Hills in Meghalaya, receives the world’s highest mean annual rainfall (around 1,187 cm) due to its unique geographic funnelling configuration, outranking neighboring Cherrapunji.
• The Coldest and Hottest Places: Dras in the Kargil district of Ladakh is recorded as the coldest inhabited place in India, with winter temperatures dropping below -45°C. Conversely, Sri Ganganagar in Rajasthan and Phalodi regularly record India’s highest summer temperatures, exceeding 50°C.
• The Role of the Somali Jet: A strong low-level cross-equatorial jet stream, known as the Somali Jet or Findlater Jet, flows off the coast of East Africa. The intensity of this jet stream directly governs the strength of the southwest monsoon winds hitting India’s western coast.
• The Western Disturbance Crop Impact: While the western disturbances cause traffic disruptions and landslides in the hills, they are economically critical for the success of Indian agriculture. The light winter rain they provide is considered liquid gold for the cultivation of Rabi crops, particularly high-yield wheat varieties in Punjab and Haryana.