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Urban Agenda | Humidity and heat: The deadly duo making summers in India’s urban centres unbearable

Jun 04, 2024 08:30 AM IST

A study reveals alarming findings about rising humidity and temperatures in six Indian megacities: Delhi, Mumbai, Kolkata, Hyderabad, Chennai, and Bengaluru.

In a rapidly warming world, record-breaking temperatures making headlines have become a recurring theme. But what has made the heat unbearable and sometimes fatal is the alarming increase in humidity along with the higher temperature.

A man takes shelter under an umbrella during severe heatwave on a hot summer day in New Delhi on May 21, 2024. Indian authorities in the capital have ordered schools shut early for the summer holiday, after temperatures hit 47.4 degrees Celsius (117 degrees Fahrenheit) with Delhi gripped by a "severe heatwave". (Photo by Money SHARMA / AFP)(AFP) PREMIUM
A man takes shelter under an umbrella during severe heatwave on a hot summer day in New Delhi on May 21, 2024. Indian authorities in the capital have ordered schools shut early for the summer holiday, after temperatures hit 47.4 degrees Celsius (117 degrees Fahrenheit) with Delhi gripped by a "severe heatwave". (Photo by Money SHARMA / AFP)(AFP)

Worse, warmer nights and a reduction in relief from the onset of monsoon are the other commonalities the Centre for Science and Environment (CSE) found in their assessment of data from six Indian megacities. Released early this week, the study titled ‘Decoding the Urban Heat Stress among Indian Cities’ compared air temperature, land surface temperature, and relative humidity of summertime temperatures in Delhi, Mumbai, Kolkata, Hyderabad, Chennai, and Bengaluru between January 2001 and April 2024 to arrive at these findings.

“Average relative humidity (RH) has significantly increased in the last 10 summers compared to the 2001-10 average. Barring Bengaluru, the average relative humidity in the decadal summertime has increased by 5-10% in the other five megacities,” the study said. This meant the heat index was rising faster than the ambient temperature in all megacities.

The study noted that even though ambient air temperature in Delhi and Hyderabad has marginally dipped, an increase in relative humidity has resulted in an overall increase in the heat index between March and August over the past two decades. While all cities have gotten hotter, the study revealed that the nature of heat that these cities were used to experiencing has changed over time. The study also confirmed the known phenomena of the urban heat island effect, where an increase in temperatures in the built-up area leads to greater heat stress, as compared to greener peri-urban areas. Further, the study found that while green covers reduce temperatures during the day, they don’t help reduce nighttime temperatures. The perfect case for this is seen in the data for Hyderabad.

While these are the broad findings of the analysis, here's the city-wise lowdown on the findings of the CSE study.

Delhi

Delhi has been facing high temperatures this summer with some weather stations in the national capital recording close to 50°C for consecutive days. Incidentally, it is also the hottest May since 2013.

Worse, the city that used to have dry heat during summer has seen an increase of relative humidity by 8% between 2001-10 and 2014-23. This high humidity has contributed to an average 3.3°C heat stress in the city even though the ambient temperature has dipped by 0.6°C decadally.

Further, the cooling of land surface temperatures at night has decreased by 9%. This phenomenon is more pronounced in the city core compared to the periphery. The study noted that while the peri-urban area cools down by 12.2°C, the city’s core cools down only by 8.5°C.

The analysis also showed that monsoons are getting more uncomfortable in the national capital, with an additional 9.4°C in average heat index during monsoons compared to the pre-monsoons.

Such high humidity levels in Delhi have made desert coolers, which were once widely used throughout the summer, ineffective. This locally manufactured cooling appliance was not a power guzzler like air conditioners. But now, with more and more switching to ACs, the power consumption this summer peaked at an all-time high of 8,302 MW.

Experts say the increased use of ACs is not sustainable. They are still unaffordable for much of the population, said Manju Mohan, the head of the Centre for Atmospheric Sciences at IIT-Delhi, suggesting minor modifications like painting roofs and walls white and adopting green roofs to reduce the impact of heat experienced indoors. “Many things like burning waste aggravate the already hot temperatures. Efforts need to be prioritised to reduce the heat, which will mean limiting emissions from all sources,” she said.

Mumbai

Unlike Delhi, Mumbai has experienced an increase in ambient temperature (by 0.6°C) and relative humidity (by 7%) between 2001-10 and 2004-23, said the CSE study. This increase in humidity has contributed to an average of 5°C heat stress in the city. The data also showed no longer any difference in the heat index during the monsoon than in the summer.

When it comes to the cooling effect of land surface temperatures experienced during the night, it has seen that go down by 24%. However, unlike Delhi, the city’s core is cooler by 3.5°C than its outskirts, while at nighttime, it’s warmer by 0.4°C than the peripheries.

A study by Arup — a UK-based sustainable development consultancy firm — released in July 2023 showed that Ghatkopar East in Mumbai was the hottest area in the city, experiencing 7°C higher temperature than its greener surroundings. In contrast, the Maharashtra Nature Park was the coolest in the city — 8°C less than Ghatkopar.

Durgesh Gupta, founder of a Mumbai-based environmental organisation Green Yatra, said that collective efforts in restoring green and blue covers by rejuvenating lakes can go a long way to make the city cooler for everybody irrespective of their socioeconomic background. “Urban forests have been seen to be cooler by as much as 5°C compared to unshaded areas. They can be accessed by all. Moreover, the surrounding areas are cooler than densely built-up areas,” Gupta said. Further, he advocated for using traditional materials for greater thermal comfort for low-income housing.

Kolkata

A 5% decadal increase in relative humidity between 2001-10 and 2014-23 is making summers worse in Kolkata even though the change in air temperature is nominal. The study said that high humidity added an average of 6.6°C of heat stress to the city, with the decadal heat index increasing by 3.5% on average.

Monsoons are much hotter compared to pre-monsoon with an average increase of heat index rise of 3.5°C. The city has also seen the number of days with a heat index of 41°C or more to triple in the last decade compared to the period between 2001-10. A notable difference between Kolkata and other cities is that Kolkata is still cooling down at night at the same rate as in the previous decade, even though the difference is only 6°C on average.

The heat island effect — where densely built-up parts of a city exhibit higher temperatures compared to their rural or greener counterparts due to the greater absorption of heat by grey surfaces — is also pronounced in the city, with the city’s core being warmer than the peripheries by 1.8°C and 1.2°C during day and night, respectively.

Ansar Khan, a climate scientist, and assistant professor at Calcutta University, said Kolkata is the most affected global city. The IPCC noted the highest increase in surface air temperature between 1950 and 2018. He said the high humidity creates a deadly combination for the majority of those who cannot afford air conditioners. The rising penetration of ACs also contributes to heating the outdoors.

“We stand at a crucial juncture where we need to take a very radical approach to cooling as the temperature is only bound to increase,” Khan said. Lauding the initiatives such as the cool roof policy adopted by Telangana, he advocated the usage of super cool materials such as a paint developed by researchers at the University of New South Wales, which was tested in a pilot in Kolkata after they proved to reduce the peak temperature by more than 5°C.

In his study, he found that the surface temperature of the painted surface was 25°C when the ambient temperature was 42°C during the daytime. He said that the government should adopt this paint for pavements, government buildings, and other paved surfaces, as is done in many global cities.

Hyderabad

The city has seen a marginal drop in decadal average ambient temperature of 0.9°C between 2001-10 and 2014-23 but the relative humidity has increased by 10% in the same period, said the CSE study.

This meant that high humidity resulted in an addition of 1.5°C of heat index in an already hot city. Unlike the other cities, the average heat index in Hyderabad in monsoons is less by 3°C than in pre-monsoons. When it comes to diurnal cooling, nighttime cooling has gone down by 13%. The urban heat island phenomena are felt stronger at night in Hyderabad. During the daytime, the city core is 0.7°C cooler than its peripheries, while at night, the core is 1.9°C warmer than the outskirts. The analysis also showed that there has been an increase in green cover in the city from 8.9% to 26.4% between 2003 and 2003, and this has shown improvement in daytime temperatures, but it has no impact on the heat experienced during the night.

Incidentally, Hyderabad, the capital of Telangana, was India's first state to adopt a cool roof policy. Anant Maringanti, urban planner and executive director of Hyderabad Urban Lab said that many government and public buildings have been retrofitted, which reduced the impact of ambient heat inside these buildings. However, he said, “The policy has not been put into action in kutcha houses and slum settlements, which need them in priority. He said rather than reflective paints and materials, insulation will work better for kutcha houses.” Maringanti said that creating areas with tree canopies and other shaded areas brings relief to residents of low-income areas who cannot afford active cooling options.

Bengaluru

This is the only city among the six where the summertime temperatures have increased marginally, relative humidity has not seen much appreciation. Decadally, the air temperature has gone up by 0.5°C, and the impact of humidity has resulted in an addition of 0.6°C of heat stress on average. In two decades, the city's heat index has increased by 2%. Bengaluru’s nights are warmer, with the nighttime cooling going down by 15%. Here too, the urban heat island effect is felt more during the night. During the daytime, the core of Bengaluru is 0.6°C cooler than its peripheries and peri-urban areas during the summer. But at night, the core of Bengaluru is 2.5°C warmer than its peripheries.

Bengaluru was known for its temperate climate and ceiling fans were good enough for summers. But now, there is a huge increase in peak power demand due to a rapid increase in air conditioner usage between March and April. However, the majority of the population in Bengaluru like in any other Indian city cannot afford ACs.

Sarah Khan, a senior associate at Bengaluru-based Center for Study of Science, Technology and Policy (CSTEP), said that strict enforcement of green building codes such as the Eco Niwas Samhita and adoption of other existing cost-effective passive design strategies and using temperature-resistant materials to build homes and other buildings can provide adequate thermal comfort without ACs. “This is just not a theory, but this has been demonstrated in practice at the Smart Ghar III Project in Rajkot under the Pradhan Mantri Awas Yojana scheme where temperatures often go north of 40°C,” she said.

Chennai

In Chennai, while there has been a 0.4°C decadal increase in ambient temperature, the relative humidity has gone up by 5% between 2001-10 and 2014-23. The contribution of humidity to the heat index is on average 6.3 °C. The heat index has gone up by 5% decadally. Like Kolkata, the days with a daily heat index of more than 41°C have tripled compared to 2001-10. There is no longer a distinction in thermal comfort between pre-monsoons and monsoons in the city. The difference in the urban heat island effect between the day and night is negligible in Chennai. During the daytime, the core of Chennai is 0.8°C warmer than its peripheries, while at night, the core is warmer by 0.9°C.

Like Kolkata, Chennai is among the worst-affected cities due to global warming. Prasanth J, the co-convenor of the Chennai Climate Action Group, said the northern part of the city, where the majority of the socio-economically weaker section lives, is comparatively more affected due to the high density of highly polluting industries. “The pollution from the industries increases the heat further in these areas. In recent years, many people have been forced to buy ACs on EMIs, which has further increased localised heating for their neighbours,” he said. He added that the heat stress in recent years has put additional financial burdens on people who anyway have meagre savings.

Already, multiple studies have documented how this recent rise in temperatures to inhospitable levels caused physical distress and reduced income and work opportunities for a majority of Indians who work in the informal sector in India.

The HT Urban Affairs team brings to you each week a story about where we live and how it affects the way we live

 

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