Indian Climate & Factors Affecting the Climate

Introduction

India's climate shows predominantly tropical characteristics though its extreme north lies in the warm temperate belt. The combined effect of latitude, topography, proximity to surrounding seas and seasonal wind reversal produces a distinctive tropical monsoon climate with a clear wet season and dry season.

• The Indian subcontinent is separated from the rest of Asia by the lofty Himalayan ranges, which block cold air masses moving southwards from Central Asia.
• As a result, during winter the northern half of India remains warmer by about 3°C to 8°C than other regions located on similar latitudes in Central Asia.
• During summer, because of the sun's position and high solar insolation in the tropics, the climate of southern India resembles an equatorial dry to tropical pattern in aspects of temperature.
• The north Indian plains are affected by a hot, dry summer wind called the loo, originating over the Thar, Baloch and Iranian deserts; it raises temperatures in these plains to levels comparable with the southern parts of the country.

• South of the Himalayas, India can be treated climatically as a tropical country; the seasonal reversal of winds over the Arabian Sea and Bay of Bengal gives India a distinctive monsoon climate.
• Hence, more precisely, the Indian climate is a tropical monsoon climate - a seasonal wet and dry climate produced by reversing pressure patterns and wind directions.

Features of Indian Climate

India exhibits high regional climatic diversity due to varied topography, wide latitudinal extent, varied distance from the seas, and marked relief differences. Major features include variability in rainfall, wide temperature ranges, and a dominant monsoon season.

Rainfall

• Most regions have distinct wet and dry seasons; however, some areas such as the Thar Desert and parts of Ladakh receive negligible wet season rainfall.
• Mean annual rainfall varies greatly. For example, Mawsynram and Cherrapunji in Meghalaya receive roughly 1,000 cm of rain annually, while Jaisalmer in Rajasthan rarely exceeds 12 cm.
• The Ganga Delta and coastal Odisha receive intense rainfall in July-August; the Coromandel Coast (southeast coast) receives little rainfall during the peak south-west monsoon months.
• Regions such as Goa, Hyderabad and Patna receive south-west monsoon rains by early June, whereas north-west India typically waits until late June or early July for monsoon onset.

Temperature

• Both diurnal (day-night) and annual temperature ranges are substantial across India.
• The highest diurnal ranges occur in the Thar Desert, and the highest annual ranges are recorded in some Himalayan areas.
• Coastal regions have the least diurnal and annual temperature variation because of the moderating influence of the sea.
• In December some Himalayan valleys may record temperatures as low as -40°C, while many coastal regions maintain averages of 20-25°C.
• Winters are moderately cold in much of the country and extremely cold in parts of the mountains; summers are extremely hot in interior plains while Himalayan summers remain moderate.

Factors Influencing Indian Climate

Several geographic, atmospheric and oceanic factors determine India's climate:

Latitudinal Location

• India's mainland extends between about 8°N and 37°N, spanning the equatorial/tropical and warm temperate zones.
• Areas south of the Tropic of Cancer receive high solar insolation and therefore experience high summer temperatures and mild winters.
• The northern parts lie in the warm temperate zone and receive lower annual insolation; however, summer heat in northern plains can be severe due to local heating and winds such as the loo.
• Coastal regions, regardless of latitude, show more moderate climates because of maritime moderation.

Distance from the Sea

• Proximity to the sea moderates temperature: coastal regions experience a maritime (equable) climate, whereas interior regions have a more continental (extreme) climate.
• Monsoon winds first reach coastal areas; hence coastal belts generally receive substantial rainfall earlier than interior lands.

The Himalayas

• The Himalayas are the most important physiographic factor influencing India's climate.
• They act as a climatic barrier between India and Central Asia, preventing the entry of frigid continental air masses during winter.
• During monsoon months, the Himalayas force moist air to ascend and split the Bay of Bengal branch into different pathways, influencing the distribution of rainfall.
• If the Himalayas did not exist, much of north India would be far drier; they help retain moisture and shape monsoon pathways into the subcontinent.

Rainfall Decrease from East to West in the Indo-Gangetic Plains

• Across the plains, monsoon rainfall generally decreases from east to west.
• Many minor low-pressure cells form across the plains during summer; as the monsoon winds move westwards, successive rainfall events progressively exhaust the moisture content.
• By the time winds reach the western plains (e.g., Delhi, Haryana), much of the moisture is already spent and rainfall is reduced.

Note: Haryana and Punjab receive winter rainfall from western disturbances; they are therefore not deserts like Rajasthan despite low summer rainfall.

Physiography and Indian Climate

Physiography strongly determines local rainfall patterns through orographic effects, rain-shadow formation and funneling effects.

(a) Rain-shadow and Semi-arid Regions

• Areas on the windward side of an orographic barrier receive heavy rainfall; those on the leeward (rain-shadow) side remain arid to semi-arid.
• For example, the south-west monsoon winds strike the Western Ghats almost perpendicularly, producing heavy rainfall on the western slopes and western coastal plain.
• Large interior areas of Maharashtra, Karnataka, Telangana, Andhra Pradesh and Tamil Nadu lie in the rain-shadow of the Western Ghats and receive scanty monsoon rainfall.

(b) Low Rainfall in Gujarat and Rajasthan

• Monsoon winds move almost parallel to the Aravalis and face no significant barrier to induce orographic rainfall; hence parts of Gujarat and Rajasthan receive low monsoon rainfall.
• During summer the region experiences mostly horizontal winds towards the low-pressure area over Tibet, with limited vertical convection in large parts of Rajasthan and Gujarat.
• In winter, sub-tropical high-pressure conditions and the Sub-Tropical Jet produce divergence, adding to aridity in some seasons.

(c) Exceptionally High Rainfall - Cherrapunji and Mawsynram

• Mawsynram and Cherrapunji are among the wettest places on earth with mean annual rainfall exceeding 1,000 cm.
• Heavy rainfall here is produced by a combination of the funnelling effect (converging moisture-laden winds channelled into narrow valleys) and intense orographic uplift.

Note: Funnelling effect refers to the concentration of moist winds into a narrow gap or valley between hills, increasing cloud density and rainfall intensity.

Monsoon Winds and Indian Climate

• The monsoon winds are the dominant control on India's seasonal climate and rainfall distribution.
• Key characteristics of Indian monsoons:
  • Sudden onset (a relatively abrupt arrival of rains following the hot season).
  • Gradual progress (monsoon advances north-westwards across the peninsula over weeks).
  • Gradual retreat (withdrawal of monsoon is slower and staged).
  • Seasonal reversal of wind direction between summer (south-west monsoon) and winter (north-east winds).
• The arrival of the south-west monsoon produces a rapid seasonal change from hot dry weather to the wet monsoon season.
• The south-west monsoon branches from the Arabian Sea and Bay of Bengal together supply most of India's annual rainfall; the north-east winter monsoon gives important rainfall to parts of the Coromandel coast and south-east peninsular India.

Upper-Air Circulation

Large-scale upper-air circulation systems - notably the jet streams - influence seasonal changes in wind patterns over India and the behaviour of monsoons and western disturbances.

Westerly Jet Stream

• The westerly jet stream blows at high speed over the sub-tropical zone during winter and strongly affects northern India.
• The southern branch of this jet stream helps steer western disturbances from the Mediterranean into the Indian subcontinent.
• Western disturbances bring winter rain, hail and occasional snow to north-western plains and the western Himalayas and are often followed by cold waves over northern India.

Atmospheric Conditions over the Indian Subcontinent in the Month of January

Easterly Jet Stream

• In summer, upper-air circulation shifts: the westerly jet weakens or moves north, and an easterly jet develops, influenced by intense heating of the Tibetan Plateau.
• The easterly jet contributes to the sudden onset and establishment of the south-west monsoon over the Indian region.

 Atmospheric  Conditions over the Indian Subcontinent in the Month of June

Tropical Cyclones and Western Disturbances

• Tropical cyclones form in the Bay of Bengal and the Arabian Sea and significantly influence peninsular India's weather, coastal hazards and rainfall distribution.
• Most cyclones originate in the Bay of Bengal and affect the eastern coast during the monsoon and post‐monsoon periods; some intense cyclones occur during October-November on the east coast.
• Fewer cyclones in the Arabian Sea tend to be smaller but can still bring significant rainfall and coastal damage along the west coast.
• Western disturbances originate over the Mediterranean and travel eastwards under the influence of the westerly jet, influencing winter weather in the Indo‐Gangetic plains and the western Himalayas.

El Nino, La Nina, ENSO and Indian Climate

Large-scale ocean-atmosphere interactions across the tropical Pacific influence the Indian monsoon and cyclogenesis. The major modes are El Nino and La Nina, often discussed together as the El Nino-Southern Oscillation (ENSO).

El Nino

• El Nino is a warm phase of ENSO when sea-surface temperatures in the eastern tropical Pacific are anomalously high.
• El Nino generally reduces monsoon rainfall over India and suppresses cyclogenesis in the Bay of Bengal, increasing the likelihood of droughts.
• However, El Nino can favour cyclogenesis in some parts of the Arabian Sea.

La Nina

• La Nina is the cold phase of ENSO with anomalously cool eastern Pacific waters and warmer western Pacific waters.
• La Nina tends to strengthen the Indian monsoon and Bay of Bengal cyclogenesis, increasing the risk of floods in India, while suppressing cyclonic activity in parts of the Arabian Sea.

ENSO (El Nino-Southern Oscillation)

• Southern Oscillation refers to atmospheric pressure changes between the western and eastern tropical Pacific.
• When the Southern Oscillation is coupled with Pacific warming (El Nino), the combined phenomenon is termed ENSO and typically results in weakened monsoon circulation over India.
• A simplified expression: ENSO = [warm water in eastern Pacific + low pressure over eastern Pacific] + [cool water in western Pacific + high pressure in western Pacific].
• Climatic impacts of ENSO events on India resemble those of El Nino in most cases: reduced monsoon rainfall, higher drought risk and altered cyclone patterns.

Summary

India's climate is fundamentally a tropical monsoon climate shaped by latitude, the moderating influence of surrounding seas, the presence of the Himalayas, physiography, monsoon wind systems, upper-air circulation (jet streams), tropical cyclones, western disturbances and global ocean-atmosphere phenomena like ENSO. These factors interact to produce strong regional diversity in rainfall and temperature, distinct wet and dry seasons, and significant interannual variability which have direct consequences for agriculture, water resources and infrastructure planning.