Climatic Regions of India – UPSC

• A climatic region has homogeneous climatic conditions, the result of a combination of factors.
• Temperature and rainfall are the two elements treated as decisive in all schemes of climatic classification. In India, rainfall varies far more than temperature, so most schemes give it greater weight.
• The whole of India has a monsoon type of climate, but the mix of weather elements reveals many regional variations. Location, altitude, distance from the sea and relief all add to them.
  • In June, Barmer can touch 48°–50°C while Gulmarg stays near 22°C.
  • Mawsynram receives over 1,100 cm a year, Jaisalmer about 12 cm in older figures.
  • Henry Francis Blanford (1889) was so struck by this range that he preferred to speak of India’s climates, in the plural.

• On this map the monsoon onset and retreat lines are the older normal dates, such as 15 July for full cover and 1 September for first withdrawal. IMD revised these dates of the monsoon in 2020.

Schemes of Climatic Regionalisation

• On the basis of variations in climate, India can be divided into a number of climatic regions. The attempts fall into two groups:

• Each scheme is judged by how closely its boundaries match soils, vegetation and farming. On this test, the scheme of Trewartha suits India well: its regions correspond very closely with the soil, vegetative and agricultural regions.

Stamp’s Climatic Regions

• Stamp built on Kendrew. The scheme is empirical, and partly arbitrary and subjective, but its limits use simple units of temperature and rainfall.
• The master line: the 18°C January isotherm divides the country in two. It runs across the root of the Peninsula, roughly along the Tropic of Cancer.
  • North of it lies temperate or continental India, with five regions.
  • South of it lies tropical India, with six.
• The role of rainfall and temperature: temperature draws the first divide. Rainfall then separates the regions within each half, while coastal and cold-mountain regions add temperature range and altitude.

Temperate or Continental India

Tropical India

• Why the rain differs:
  • Region 6: the Bay branch meets the Meghalaya hills, where older figures give Cherrapunji 1,102 cm and Mawsynram 1,221 cm.
  • Region 7: the Bay branch and Bay cyclones. Totals thin out westward and southward.
  • Region 8: it lies behind the Western Ghats, in their rain shadow.
  • Regions 9 and 10: the Arabian Sea branch. On the Malabar Coast the wet months run to about nine.
  • Region 11: the retreating monsoon from the north-east, mostly in November and December.
• Evaluation:
  • Strength: easily understood limits, and a clear link with the real pattern of rain.
  • Weakness: the boundaries are lines drawn on judgement, with no test for how effective the rain is.
  • The transition from Region 4 to Region 5 shows that the climate changes gradually, while a map gives a line.

Trewartha’s Classification of Climate

• Glenn Thomas Trewartha modified the classification of Köppen and presented it in 1954. His scheme is empirical, based on temperature and precipitation data, and he also used English letters as symbols.
• World types: A tropical humid, B dry, C humid mesothermal, D humid microthermal, E boreal, F polar, with H for undifferentiated highlands.
• Applied to India, the scheme gives four major climatic regions (A, B, C, H), further divided into seven meso-climatic divisions:

• Tropical Rainforest Climate (Am):
  • High temperature and heavy precipitation.
  • Dense forests, chiefly of evergreen trees, are the characteristic vegetation.
• The Tropical Savannah Climate (Aw):
  • It has a marked dry season, and nearly all the rain falls in summer.
• The Tropical Steppe Climate (BS):
  • It covers Peninsular India east of the Western Ghats, the rain-shadow area in parts of Maharashtra, Telangana, Andhra Pradesh, Karnataka and Tamil Nadu.
  • Rayalaseema belongs here. Its low, unreliable rain and high temperatures follow from its place behind the Ghats, with too little of either monsoon to make up the gap.
• The Sub-tropical Steppe Climate (BSh):
  • It is semi-arid, and its annual range of temperature is significantly high.
• The Tropical Arid Climate (BWh):
  • Thorny bushes make up the natural vegetation.
  • Older accounts record the country’s lowest rainfall in Ganganagar district. Jaisalmer and the cold desert of Ladakh are at least as dry.
• The Humid Subtropical Climate (Caw):
  • Winters are dry, and western disturbances add light rain in the west.
• Mountain Climate (H):
  • Mean temperatures for all months depend closely on topography and slope.
  • The Western Himalayas get some rain from western disturbances in the winter season.

• Key to the letters: A Tropical climate, B Dry climate, C Humid subtropical climate, D Humid temperate climate, E Polar climates, H Highlands.
• The legend of this map needs care. Its “sub-tropical humid (dry winter)” belt over central India is the tropical savannah (Aw) of the text. Its “sub-tropical savanna (dry summer)” in Tamil Nadu is the tropical As type with its winter rain.

Other Schemes

Köppen’s Scheme in Brief

• Wladimir Köppen first published his world scheme around 1900 and revised it until 1936. Its classification system uses monthly and annual means of temperature and rain, and treats native vegetation as the truest sign of a climate.
• Five major types apply worldwide: A tropical rainy, B dry, C mild mid-latitude, D severe mid-latitude, E polar.
• Small letters add detail: a hot summer, c cool summer, f no dry season, w dry winter, s dry summer, g Ganges-type hot season before the solstice, h hot and dry (mean above 18°C), m monsoon with a short dry season.
• Nine regions are usually mapped in India:

• Limit: it uses no direct measure of evaporation, and its fixed limits sit poorly on a monsoon country with sharp relief.

Thornthwaite’s Scheme

• Charles Warren Thornthwaite published his classification in 1931, and a markedly different one in 1948.
  • 1931: the plant is a meteorological instrument. Precipitation effectiveness (12 monthly P/E ratios added together) and thermal efficiency (T/E) set the types. The Indian map below uses this version.
  • 1948: vegetation becomes the means by which water passes from soil to air, and potential evapotranspiration replaces the old indices, the core of Thornthwaite’s classification.

• Seasonal letters: r adequate rain in all seasons; s summer deficient; w winter deficient; d deficient in all seasons.
• Of 120 possible combinations he mapped 32 on the world map. Twelve occur in India:

R. L. Singh’s Climatic Divisions

• Singh (1971), following Kendrew and Stamp, set ten divisions by the temperature of the hottest and coldest months and by annual rain. It keeps close to India’s relief and to the humid-to-arid gradient.

Comparing the Schemes

• The same logic applied at a finer scale for farming gives the agro-climatic zones. The wider picture appears under the world’s climatic regions.