As Indian cities expand and grapple with alternating severe droughts and devastating floods, researchers have uncovered how rainfall patterns have transformed across 63 major urban areas in Peninsular India over the last 120 years. In a new study, scientists from the Indian Institute of Technology (IIT) Hyderabad and the University of Colorado Boulder, USA, analysed daily rainfall data from 1901 to 2022 to understand the shifting climate. They discovered that cities are experiencing four highly distinct, localised patterns of change, ranging from recent, sudden surges in extreme downpours to steep declines in annual rainfall. By calculating these trends using rolling blocks of time, the team has shown that urban planners must adapt to fluid, evolving weather cycles rather than relying on outdated historical averages, ultimately helping to safeguard millions of residents against future climate extremes. 

The researchers, rather than calculating simple century-long averages, employed a statistical technique known as change-point analysis. Using this tool, they could scour decades of data to pinpoint the exact years where abrupt shifts in a weather system occurred. The team found that the most dramatic disruptions in annual and monsoon rainfall occurred primarily between the 1950s and the 1970s. For example, cities along the southwestern coast have seen a stark decrease in both annual and monsoon rains, threatening water availability. Meanwhile, hubs like Bangalore have experienced sudden, sharp increases in maximum extreme rainfall in recent decades, heavily raising the risk of severe urban flooding. 

To understand how these changes unfold, the team used a moving window approach. Instead of calculating a single trend for the entire 122-year period, they analysed the data in sliding chunks of 30 to 55 years, advancing by one year at a time. This allowed them to see how the trends themselves evolved. They then used a clustering algorithm to group cities with similar rainfall behaviours, proving that even within the same region, neighbouring urban areas can face wildly different climate futures.

Historically, scientists would take a fixed, long-term block of time and calculate a single straight-line trend. However, that traditional method often masked shorter, multi-year climate cycles where rainfall might dip dramatically for a few decades before surging again. By applying these moving windows and focusing specifically on the footprints of dense urban agglomerations rather than broad, vaguely defined regional climate zones, the researchers provided a much more accurate, high-resolution picture of the specific hazards each city faces. 

However, the researchers note that the study relies on gridded rainfall data provided by the India Meteorological Department. Because this data mathematically averages rainfall over a spatial grid, it can sometimes smooth out and underestimate highly localised, extreme weather events. Furthermore, the researchers analysed the cities based on their modern, fixed geographic boundaries, meaning the study does not account for how the physical expansion of these urban environments over the century might have uniquely altered local rainfall dynamics. 

Nonetheless, the research provides a roadmap for the survival of rapidly growing metropolitan areas. By proving that rainfall variability is dynamic and highly specific to individual cities, the work gives urban planners the evidence needed to design adaptable infrastructure. Whether it means building larger stormwater drains for cities facing sudden extreme downpours or establishing strict water conservation policies for those experiencing creeping droughts, this research ensures that future development can be built to withstand the complex realities of a changing climate.