It's hot in Kochi and even winter may not bring some relief. Barring rainy days, Kochi now swelters year-round and a recent study on Urban Heat Island (UHI) and Land Surface Temperature (LST) confirms this troubling shift. Kochi’s winters (November-December) are heating up faster than its summers (March-April). Between 2000 and 2023, the study found, summer LST rose by 0.07°C annually, while winter LST climbed by 0.08°C per year. This amounts to a total increase of 1.7°C in summer and 1.9°C in winter over two decades. While Urban Heat Island effect refers to the phenomenon where cities become significantly warmer than nearby rural areas, Land Surface Temperature, or LST, reflects how hot the ground feels to the touch.

Published in the International Journal of Environment and Climate Change, the study conducted by Chandu P J and Bindu G of the Nansen Environmental Research Centre, and Vishnu Sharan K R of Kerala Agricultural University compares seasonal urban heat patterns in Kochi, Kerala, and Fairbanks, Alaska.

Over the past decade, Kochi has exhibited a consistent warming trend in both summer and winter. In contrast, Fairbanks recorded summer warming but a slight cooling during winter. Notably, Kochi experienced a sharper increase in Urban Heat Island (UHI) intensity during winters than in summers. In 2014, the city’s average winter UHI intensity was 0.6°C; by 2023, it had climbed to 1.4°C. The researchers link this to rapid urbanisation and a steep decline in green cover, especially in central and newly developed areas of the city.

“UHI intensity is higher in winter because rural areas cool down faster, aided by natural mechanisms like sea breezes. In contrast, urban areas retain heat due to construction, land-use changes, and anthropogenic heat sources,” explains Chandu P J. “For example, as we exit Kochi city via the Kundanoor bridge, we can physically feel a drop in temperature-that’s the effect of the heat pocket.”

One of the study’s most striking findings is the threefold increase in high-UHI zones (areas with UHI intensity over 1.5°C) during winter. These zones expanded from 14 km² in 2014 to 41 km² in 2023. At the same time, cooler zones (with negative UHI values) shrank from 20.3 km² to just 2 km². On an average day, the difference between high-UHI and low-UHI zones now ranges from 1.5°C to 2°C.

According to the researchers, the increase in UHI and LST is directly correlated to the increased construction and lower green cover. “Most of the areas with high UHI have shown increased construction work and reduced green cover over the years. Spatially, high-UHI zones in summer include Fort Kochi, Willingdon Island, and the High Court area, while in winter, Kalamassery records the highest UHI intensity. The variations across seasons are driven by changes in wind flow and other factors,” says Chandu.

The study warns that Kochi’s changing heat profile--driven by high humidity, sustained warming, and dwindling green infrastructure--is intensifying thermal discomfort, especially during months that were once relatively cool. Air pollution, fuelled by the rising use of air conditioners and vehicles, worsens the situation, trapping the city in a vicious loop. These shifts, the researchers note, have serious implications for public health, energy consumption, and future urban planning.

To address this growing crisis, the authors recommend targeted interventions: expanding green cover and permeable surfaces, promoting vertical gardens, designing heat-resilient infrastructure, and integrating climate data into zoning laws and building regulations.