Pune: The city’s most densely built-up neighbourhoods experienced extreme heat stress on nearly half the peak summer days during April and May 2024, showed a detailed field analysis conducted across three locations using the Wet Bulb Globe Temperature index — a composite indicator that combines air temperature, humidity, wind speed and solar radiation.The on-ground assessment was carried out by scientists from the India Meteorological Department (IMD) in collaboration with the Indian Institute of Science Education and Research (IISER), Fergusson College and the Indian Institute of Tropical Meteorology (IITM).“Unlike standard temperature readings, the Wet Bulb Globe Temperature (WBGT) index captures the actual heat load on the human body by including radiant heat and humidity. Sensors were placed at four feet — typical human exposure height — and logged data every five minutes between 9am and 6pm,” IMD scientist Ananya Karmakar said.Among the three monitored sites, Fergusson College (categorised as a highly urbanised zone in the heart of the city) recorded extreme heat stress conditions on 44.18% of the 43 days, for which data was available. In comparison, the Agriculture College campus, classified as moderately urbanised, recorded 37.03% extreme-stress days, while the greener IISER campus on the city’s outskirts recorded 32.72%.“The findings point to a clear urban heat island signal, where dense infrastructure, concrete surfaces and limited vegetation intensify heat exposure. The Fergusson College site, surrounded by heavy construction and high human activity, consistently recorded higher daily maximum WBGT values,” IITM scientist Rajib Chattopadhyay said.To allow an equal baseline for comparison, researchers also examined the 39 days with complete datasets across all stations. Even in this subset, Fergusson College showed the most extreme heat‑stress days at 43.58%, followed by Agriculture College campus (35.89%) and IISER (30.76%).When scientists assessed typical peak heat-stress levels using the median daily maximum WBGT, the urbanised Fergusson College area again topped the list at 31.87°C, compared with 31.70°C at Agriculture College campus and 31.50°C at IISER. In simpler terms, even on a typical hot day, the densest urban pocket remained slightly but consistently more heat-stressed than the greener campuses.Across all sites, heat stress peaked between 1pm and 3pm. Fergusson College and Agriculture College campus recorded their highest values around 2pm, while IISER — with more vegetation and open spaces — peaked earlier, around 1pm, indicating that greener landscapes heat and cool more quickly than built-up zones.IITM scientist Chattopadhyay said, “The analysis also established preliminary heat stress trigger values for Pune during April and May (2024). A WBGT temperature of 31.5°C was identified as the threshold for elevated risk, 32°C for moderate risk and 33°C for extreme heat stress. These values corresponded roughly to ambient temperatures of 40°C, 41°C and 42.5°C, respectively, depending on humidity levels.” He, however,said analysis was a preliminary study based on one-year field experiment. “Multiple year data is required to understand the impact of urbanization on heat stress in the Pune City,” the IITM scientist said.Air temperature turned out to be the main factor behind heat stress — as temperatures rose, heat stress levels increased sharply too. Humidity showed the opposite daily pattern, increasing when heat stress was lower and dropping when it was highest. Wind also played an important role: stronger winds helped cool conditions by carrying heat away from the body, reducing overall heat stress. The most intense heat stress, typically between 1pm and 3pm, was linked to winds blowing from the south.The field analysis was led by Ravi Ranjan Kumar, Arpit Tiwari and Ananya Karmakar of IMD, Pune, along with Ajay Bankar, Rajib Chattopadhyay and KS Hosalikar. The team also included Joy Merwin Monteiro from IISER Pune and Shayanth Patil from Fergusson College. The findings have been published in the peer-reviewed journal International Journal of Biometeorology.
