Measuring the human impacts of extreme heat to guide cities’ climate action plans

Two women and a man standing in front of an auto-rickshaw on a city street.

Anu Ramaswami with Dhanasree Durairaj, a local Chennai intern with Princeton University, and Amir, an auto- rickshaw driver. Photos courtesy of the researchers

Last May, Neha Agarwal returned to her home city of Delhi, India, to initiate a Princeton study measuring human exposure to extreme heat. Stepping out of the airport “felt like walking into a furnace,” said Agarwal, a Ph.D. student in civil and environmental engineering.

When Delhi hit an all-time high temperature of 120.4°F (49.1°C) on May 29, Agarwal was distributing pendants to track individuals’ heat stress over a 24-hour period. She also asked residents what was helping them endure the heat.

“One participant said that when they wash clothes and they have some gray water, instead of just draining it out, they splash it onto their roof. It immediately evaporates” into Delhi’s dry air, “but at least for a brief moment, they have some experience of cooling,” said Agarwal.

Agarwal is a member of Princeton’s Urban Nexus Lab, led by Anu Ramaswami, Sanjay Swani ’87 Professor of India Studies and a professor of civil and environmental engineering and the High Meadows Environmental Institute, as well as director of Princeton’s M.S. Chadha Center for Global India. Ramaswami designed the study to focus on measuring human exposure to extreme heat stress in India’s cities, with potential to extend learning to other cities of the Global South.

Three researchers affix instruments to a car parked on a tree-shaded street. — Princeton undergraduate interns Chaeyoung Lee and Seungho Lee work with graduate student Neha Agarwal to attach sensors to a car in Chennai

Ramaswami’s lab aims to improve cities’ equity and environmental sustainability in tandem, taking a holistic approach to solutions in energy, buildings, transportation, and other sectors. With extreme heat becoming more frequent, the variability in exposure to heat stress across socioeconomic groups and neighborhoods is a missing piece of this complex puzzle.

In Delhi, the team gave wearable pendant heat sensors to a cross-section of residents with diverse dwelling types, occupations, and commuting patterns. The pendants use temperature and humidity measurements to calculate the heat index.

Earlier in the summer, Associate Professional Specialist Ajay Nagpure initiated a parallel study in Chennai. There, daytime temperatures hovered around 98.7°F (37°C), which sounds bearable until you factor in the air’s 80% humidity, which can make it feel about 50°F warmer.

In addition to the wearable pendants and other air temperature sensors to measure variations across neighborhoods, the team brought with them a novel cube sensor designed by Forrest Meggers, associate professor of architecture and the Andlinger Center for Energy and the Environment, to measure the heat radiated by surfaces, a key component of thermal comfort that’s not captured by air temperature.

Other Princeton collaborators include professor Elie Bou-Zeid, an expert in urban microclimates who will apply models to predict the effects of different interventions; and assistant professor Jyotirmoy Mandal, whose group creates optical materials that can cool buildings and neighborhoods by reflecting sunlight and radiating heat to the sky.

Heat is among the challenges that Ramaswami’s group is addressing through a new partnership with the Chennai Metropolitan Development Authority. Similar to collaborations Ramaswami has forged with U.S. cities, her group plans to work with policymakers in Chennai to model pathways to decarbonization that offer co-benefits to health, climate resilience, and quality of life. The work in India is supported by the M.S. Chadha Center for Global India.