IMD Study: Southern India Has Become Wetter and North-West India Drier Over Seven Decades
A study by scientists of the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune, found that southern India has become wetter between 1951 and 2020.
The area of the peninsula with a "hot semi-arid" climate (a dry, grassland type climate) has nearly halved: from about 30% of the peninsula in 1951-1980 to about 16% in 1991-2020.
Most of this land has moved into the "tropical savanna" climate zone, which gets more rain but still has a long dry season. Savanna now covers nearly three-fourths of peninsular India, up from about three-fifths in the 1950s.
North-west India moved the other way. It became drier and hotter, and the hot semi-arid zone spread from western to eastern Uttar Pradesh as rainfall declined. Day and night temperatures rose across the country.
The study used the Köppen-Geiger climate classification and IMD rainfall and temperature records for 1951-2020, compared over successive 30-year periods. It was published in MAUSAM, the IMD's peer-reviewed journal.
The researchers said these shifts matter for choosing crops and sowing dates. The study comes as the northeast monsoon, which Tamil Nadu depends on, set in on October 15, 2026, five days before its normal date.
Köppen Climate Classification
The Köppen climate classification is the world's most widely used system for sorting places into climate types. It uses just two things that anyone can measure: temperature and rainfall, month by month. Based on these, every place on Earth gets a short letter code, such as Aw (tropical savanna) or BWh (hot desert). Geographers, ecologists and agricultural planners use it to compare climates and to see how they are changing.
The IMD-IITM study applied the Köppen-Geiger system to India's meteorological subdivisions over successive 30-year normals from 1951 to 2020. It found that much of the peninsula's BSh (hot semi-arid) area has crossed into Aw (tropical savanna) because rainfall rose, while north-west India's semi-arid belt has widened. These label changes are a simple signal that India's regional climates are shifting.
Northeast Monsoon: India's Winter Rain Season
The northeast monsoon is the rainy season that comes to the south-eastern part of India in October, November and December. It is called "northeast" because the winds blow from the north-east towards the south-west at this time of year. For India as a whole it is the smaller monsoon, but for Tamil Nadu and nearby areas it is the main rainy season. When people in Chennai talk about "the monsoon", they usually mean this one.
The IMD study links the long-term rise in peninsular rainfall to its shift from semi-arid to savanna climate, and the lead author stressed this for Tamil Nadu because the state depends on the northeast monsoon. In 2026, the northeast monsoon set in on October 15, five days early, and the IMD forecast a normal season for Tamil Nadu (seasonal average about 44 cm) after a 20% deficit in the southwest monsoon.
- Peninsula's hot semi-arid area: about 30% (1951-1980) to about 16% (1991-2020)
- Tropical savanna: about three-fifths of the peninsula in the 1950s, nearly three-fourths now
- Data period: 1951-2020, in successive 30-year periods; most of the southern shift came in 1981-2010 and 1991-2020
- North-west India: hot semi-arid zone spread from western to eastern Uttar Pradesh; rainfall declined
- Method: Köppen-Geiger classification across IMD meteorological subdivisions; published in MAUSAM (IMD journal)
- Northeast monsoon 2026 onset: October 15 (normal date about October 20)
- Tamil Nadu northeast monsoon seasonal average: about 44 cm; Tamil Nadu southwest monsoon 2026: 20% deficit; all-India southwest monsoon 2026: 12.6% deficit, with El Niño conditions