Bengaluru set for the wettest year; experts say it’s Urban Heat Island effect

BENGALURU: Due to unprecedented rain in the past few months, Bengaluru is inching close to setting an all-time record for the wettest year in 75 years.
The previous wettest year was 2017 when the city received 169.6cm of rainfall, according to data from India Meteorological Department (IMD).

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Scientists at IMD Bengaluru told TOI that rainfall in the city is measured at three locations – Bengaluru City, HAL Airport, and Kempegowda International Airport. “Bengaluru has so far recorded 169cm of rainfall (January 2022 to October 17, 2022). The city still requires 0.7cm to break the previous record for the wettest year set in 2017. But with two and a half months remaining, 2022 will be the wettest year for Bengaluru since the recording of climate data began,” said Geeta Agnihotri, scientist and head of IMD Bengaluru. The department has data for Bengaluru rain from 1947.

In October alone, the city has so far received 22cm of rainfall. “Generally, October marks the end of southwest monsoon for Bengaluru, and northeast monsoon usually sets in by October 15-20, resulting in rainfall in Kolar, Bengaluru Urban, and Ballari. This year, the onset of a retreating monsoon is expected in a few days, which will bring considerable rain to the city. If there is any depression over the Bay of Bengal, the city will get more rain,” Agnihotri explained.

Dispelling the notion that excess rainfall is due to climate change, meteorologists explain that the global phenomenon has nothing to do with the yearly increase in rainfall as it is largely the result of ‘micro climatic’ factors.

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“It is mainly local factors ranging from change in the landscape to loss of green area that is influencing the rainfall pattern in Bengaluru. World over, this has been the trend in several developed cities due to the rampant change in the land-use pattern and creation of heat islands,” Agnihotri clarified.
Scientists from the Divecha Centre for Climate Change (DCCC) at the Indian Institute of Science (IISc) pointed to local factors for the phenomenon.

Prof J Srinivasan, the distinguished scientist at DCCC, said annual rainfall in most tropics is increasing largely due to an increase in water vapor in the atmosphere triggered by the rise in global mean temperature.

“In Bengaluru, for example, the temperature increase is both on account of global warming and the Urban Heat Island effect. Today, the urban regions of Bengaluru have a lot of built-up areas that trap the heat from the Sun and hence the nights are warmer. As the evenings are warmer in Bengaluru than the surrounding areas, there is an increase in the upward motion of the air, leading to the formation of clouds and increased rainfall towards the evening or late evening,” Srinivasan explained.

He added until the middle of the 20th century, rainfall in Bengaluru varied mainly on account of changes in surface temperature in the tropical Pacific (El Nino or La Nina). “Changes in the Pacific Ocean alter the global circulation pattern, affecting rainfall all over the world. This is called natural variability,” Srinivasan opined.

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