All through April and May this year, as the maximum and minimum temperatures soared in Pune, a team of scientists and students from the India Meteorological Department (IMD), Fergusson College, and the Indian Institute of Science Education and Research (IISER), were performing detailed measurements to quantify the ambient heat in a manner that is directly relevant to humans.

Their data went beyond the air temperature. It was the wet-bulb globe temperature (WBGT) that the team focused on.

According to the IMD, WBGT is one of the most commonly used indices by many organisations since the 1960s. “This index was originally invented in the 1950s in efforts to lower the risk of heat disorders during the training of the US Army and Marine troops. Since that time, WBGT has been applied in other settings, and is widely used for the evaluation of occupational heat stress exposure,” says IMD.

A high temperature almost definitely means it’s dangerous, but dangerous to whom and to what extent – this was what the study sought to measure. “The maximum temperature is useful as an indicator, but it is not particularly useful as a decision making number or a quantity. The reason WBGT is used is it has published thresholds for danger levels,” says Dr Joy Merwin Monteiro, Assistant Professor, Department of Earth and Climate Science, IISER Pune.

India is one of countries outside the global north that have not calibrated WBGT to their population. In the US, a WBGT of less than 26.7 degree Celsius (approx) means that new athletes can undertake unlimited activity with primary cautions or extreme exertion but must schedule mandatory rest or water breaks. On the other end of the spectrum, an intense WBGT reading of more than 32.2 degree Celsius (approx) indicates that all athletes must suspend practice or have mandatory breaks as directed by the sports administration.

In February this year, the collaborative team across IMD, FC and IISER, led by Rajib Chattopadhyay, published the first-ever study on WBGT in the city, “Quantifying heat stress using wet bulb globe temperature measurements during summer 2024 from field experiments in Pune”.

For the study, three locations were chosen to install WBGT meters –IISER, Fergusson College (FC), and Agriculture College. During the peak summer months of April and May 2024, the research team carried out readings every hour, every day between 9 am and 6 pm. “It was found that maximum heat stress usually occurred between 1 pm and 3 pm, with variations depending on the location,” says the study.

The findings

The findings from the study established the WBGT threshold limits for Pune. These were 31.5 degree Celsius, 32 degree Celsius and 33 degree Celsius. “This corresponds to low, moderate, and extreme heat stress levels,” says the study. While the researchers found that a strong positive correlation between WBGT and ambient temperature, relative humidity and wind speed had a reverse correlation. “Notably, southerly winds contributed most significantly to heat stress,” says the study.

According to Monteiro, the findings equip policymakers, urban planners and environmentalists with essential insights. The data can be used as foundations to decide strategies to mitigate the challenges of climate change and enhance urban resilience against heat stress.

Monteiro says that it is essential to understand heat stress in a more holistic manner, and using WBGT instead of just air temperatures is an important step in this direction. This holistic measurement is particularly important in the context of the health of the elderly and outdoor workers, among others. “If these measurements are carried out in a larger scale across different urban morphologies, it can also quantify how urban design and vegetation will influence heat stress level,” says Monteiro.