Weird Science | Let’s settle the eternal Mumbai v Delhi argument - Hindustan Times

Weird Science | Let’s settle the eternal Mumbai v Delhi argument

Dec 08, 2023 03:36 PM IST

Delhi residents, facing lower temperatures, scoff at ‘winter’ in Mumbai. Why do residents of the coastal city and the northern city seem to feel equally cold?

Winter is yet to descend upon us in all its glory, but Delhi and Mumbai residents have already acknowledged the season with the same attitude even though the latter is some 10 degrees warmer than the former. Delhi’s minimum temperatures ranged between 10 to 15°C, while the maximum hovered around 25-27°C over the first few days of December. Mumbai, in contrast, recorded temperatures in the late 20s and early 30s. In other words, its highest was not too far from Delhi’s lowest.

People take a stroll at Kartavya Path amid dense fog on a cold winter day in New Delhi. (ANI Photo) PREMIUM
People take a stroll at Kartavya Path amid dense fog on a cold winter day in New Delhi. (ANI Photo)

Yet, residents of both two cities appear to feel as warm or as cold as one another.

In a general sense, perhaps 20°C evokes the same response in a Mumbai resident as 15°C might in someone accustomed to living in Delhi. Why?

An intuitive way to explain this is habituation: The way people perceive hot and cold depends on the climate they are accustomed to. There is, however, a lot more involved in thermal comfort: Temperature, clothing, humidity, and air speed, and, studies show, that even an individual’s socioeconomic status and gender play a role.

What is thermal comfort?

Thermal comfort quantifies our satisfaction with the thermal environment. It takes into account several factors, some of these being individual characteristics (metabolic rate and insulation provided by clothing) and some relating to the thermal environment (air temperature, radiant temperature, air speed and humidity).

“All these factors are combined to determine the thermal neutral temperature preference of the occupants, i.e. at what temperature occupants will not prefer either a hotter or a cooler thermal environment,” Professor Ronita Bardhan, an architectural engineer and urban studies educator at the University of Cambridge, said over email. She has done extensive research on thermal comfort, including in indoor settings in Mumbai.

Using these variables and various methods, a thermal comfort zone can be determined to express what environmental conditions are acceptable to an individual. For given values of humidity, air speed, metabolic rate, and clothing insulation, the thermal comfort zone is expressed in terms of a range of temperatures.

This, of course, varies among individuals in different regions. People tend to show a preference for a certain comfort temperature based on acclimatisation (thermal history and access to cooling and heating appliances) and habituation (psychological adaptation based on social norms and cultural practices), Bardhan said.

“Studies in the global north suggest that summer comfort temperature ranges between 23–26.1°C and winter comfort zone between 20–24.5°C. In India, according to the National Building Code 2016, a thermally comfortable temperature is assumed to be 26ºC for cooling and not less than 18ºC for heating. Thus people start to feel discomfort beyond 26ºC and will seek interventions like mechanical cooling in summer/hotter areas,” she said.

“However,” she added, “our studies show that thermal comfort is also dependent on people’s socioeconomic status and gender.”

Other facets of comfort

Studies by Bardhan and colleagues have found higher thresholds of thermal comfort in India, within a larger range of 23.5-32°C. People in lower-income groups, again, have a higher comfort acceptability of the environment than higher-income groups; they report being comfortable even at 28.7°C.

“We have also found different thermal comfort thresholds for females and males. Females in lower-income communities, who generally spend 40% more time indoors than men, tend to have higher adaptive saturation and refrain from using energy-intensive cooling appliances for their thermal comfort, and only operate for the children and male occupants’ needs,” Bardhan said.

She related this to the fact that women face higher social expectations, to be more resilient than men. “This is related to social definitions of ‘good housewife, or good mother’.”

There is also a limit to the levels of temperature and humidity that the human body can endure. This is quantified in terms of wet bulb temperature, or the lowest temperature to which an object can cool down when moisture evaporates from it.

“A wet bulb temperature of 35°C is considered the theoretical human survivability limit with up to six hours of exposure. This means that at 100% humidity and dry bulb temperature (room temperature) of 35°C, humans will be unable to sweat any longer and keep themselves cool,” she said.

Once that stage is reached, no measures will work. Whether one switches on the fan, drinks lots of water, or stays in the shade, the body will not physiologically recover from heat stress.

In extreme climates, this can be especially problematic.

Bardhan referred once again to Indian women from low-income communities who have a high thermal comfort threshold and do not take any interventions when it is very hot.

“Prolonged exposure to elevated temperatures may lead to respiratory and cardiovascular conditions. Additionally, recent research indicates that wet bulb temperatures exceeding 35°C can impact the immune system, which means that extreme heatwaves could push climate beyond human endurance and cause severe health burdens.”

Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but it isn't

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    Puzzles Editor Kabir Firaque is the author of the weekly column Problematics. A journalist for three decades, he also writes about science and mathematics.

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