Heat Zone Delhi

New CSE study on the summer that is about to end in Delhi says climate change is making the city so hot and humid that it is not cooling down adequately even during nights, adding to severe thermal discomfort 

As a result, demand for electricity – driven largely by mechanical cooling (especially air-conditioners) -- is surging. In 2023, average daily peak electricity demand in the monsoon period was the highest since real-time
load monitoring started in Delhi in 2018

  • Delhi’s summer heat patterns undergoing a change -- humidity rising faster than the ambient temperature.
  • Days with high heat index -- when both temperature and humidity are high – rising: mostly during monsoon period when high humidity adding 5-7°C heat load on the city
  • City not cooling down even at night – growing public health risk.
  • Hotter and muggier nights driving cooling and electricity demand. Daily peak electricity demand occurring around midnight on most summer days.
  • Average night-time peak electricity demand during monsoon almost 40 per cent higher than average pre-monsoon peak.
  • Every degree rise in heat index leads to 140-150 MW increase in daily electricity demand and 190-200 MW increase in the night-time electricity demand due to higher cooling requirements during night. 
  • Heat Action Plans need to go beyond emergency response during extreme summer day-time heat; should address all heat generators including waste heat from exhaust of building cooling systems, vehicles and industry, and heat-absorbing concrete surfaces that contribute to heat island effect. 

You can access the detailed analysis click here 

New Delhi, October 27, 2023: Climate change and extreme weather has started exacting a heavy toll in India’s national capital. The local impact of climate change and extreme weather events is now starkly evident in the unique trends in humidity and ambient temperature of Delhi, as witnessed in the summer that is just passing by. Humidity levels are rising faster than ambient temperatures, adding to severe thermal discomfort for city dwellers – this is leading to a spike in cooling requirement and electricity consumption in Delhi. Even the nights are not cooling down adequately during summer and monsoon. 

This has emerged from a new analysis carried out by Centre for Science and Environment (CSE) of the trends in heat, humidity, land surface temperatures and resultant electricity demand during the summer and monsoon months in Delhi. 

CSE researchers point out that these local trends are consistent with the global observations made by the Intergovernmental Panel on Climate Change (IPCC)’s Sixth Assessment Report (AR6 WG-I). The report has said that hot extremes, including heat waves, have intensified in urban centres that are experiencing air temperatures several degrees warmer than the surrounding areas, especially during the night.

Urban heat island effect can add 2°C to local warming, reducing the adaptive capacity of cities and increasing risks, warns the IPCC. 

“It is important to track the impact of heat trends in cities; these trends can compromise adaptive thermal comfort of people and increase the demand for active cooling-air conditioning, affecting electricity demand and energy security in the process,” says Anumita Roychowdhury, executive director, research and advocacy, CSE. 

“These trends need to be understood and considered for improving city-wide heat management as well as for introducing better building design and materials to reduce the heat load and improve adaptive thermal comfort,” Roychowdhury adds. 

The Delhi government’s newly announced heat action plan needs to go much beyond emergency response during extreme heat waves to more sustained and systemic action to address all sources of heat, including waste heat from buildings, vehicles and industries, heat-absorbing concrete and urban building materials and infrastructure,” says Roychowdhury. 

According to Avikal Somvanshi, senior programme manager, Urban Lab, CSE: “Public health emergency action needs to consider both high day-time and night-time temperatures and relative humidity. The heat problem is not only about daily maximum temperatures crossing the 45°C benchmark that has become the standard focus during summers – it involves a much more complex set of indices.” 

“Urban heat mitigation also requires more robust scientific tracking of the key indicators. These include -- besides ambient heat and temperature -- surface heat absorption and land surface temperatures, changing land-use including vegetative cover, and waterbodies that are determinants of the heat island effect. This requires effective leveraging of available satellite technology; we have the data – what is needed is policy integration,” adds Somvanshi.   

Methodology behind the analysis

The CSE study is based on comparative statistical analysis of temperature and humidity conditions observed in Delhi for the period March to August 2023. The study period has been furthered divided into pre-monsoon (March-May) and monsoon (June-August) seasons, as per the India Meteorological Department (IMD) classification. 

The analysis has used publicly available datasets from various national and global agencies. Ambient temperature and humidity data is sourced from IMD weather stations at Palam and Safdarjung in Delhi – an average of the two weather stations has been used to represent the city. Heat Index computation has been done using the US National Oceanic and Atmospheric Administration’s (NOAA) formula. Complex geospatial calculations have been done in Python and ArcGIS. 

Freely accessible MODIS Land Science data from NASA Earth Observations has been used for seasonal land surface temperature averages. For the analysis of specific extreme heat days, Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager/Thermal Infrared Sensor (OLI/TIRS) satellite imagery have been used to determine the land surface temperature, land use patterns and normalised difference vegetation index (green cover). 

Power demand data has been sourced from the real-time data portal of the State Load Dispatch Centre in Delhi. 

Key findings of the CSE study 

Seasonal ambient temperature is stable, but the city weather is becoming more humid: Summer-time seasonal daily mean ambient temperature has been quite stable since 2011, finds the CSE study. The summer of 2022 was the hottest, with a seasonal average of 31.2°C; the summer of 2023 has been least warm, with a seasonal average of 28.9°C. This was largely because of the unseasonal rains that the city received in summer. 

The decadal average of last 10 summers is marginally lower than the decadal average for 2001-10 -- but this does not mean Delhi is getting cooler. The changes in the pattern of rains, especially unseasonal rains in the pre-monsoon period, have led to an increase in relative humidity which is making the city’s weather muggier and more uncomfortable despite the fact that there has been no significant changes in ambient temperatures. 

Average relative humidity (RH) has increased significantly in the last 10 summers compared to the 2001-10 average: During the pre-monsoon season in 2023, the average RH stood at 49.1 per cent – this was about 21 per cent more humid than the 2001-10 average. During monsoon, the average RH was 73.2 per cent: 14 per cent higher than the 2001-10 average. This rise in humidity compared to 2001-10 has been observed throughout the study period starting 2018. 

Says Somvanshi: “This combination of high heat and humidity can compromise the human body’s main cooling mechanism: our propensity to sweat, which cools our bodies. Higher humidity limits this natural cooling. As a result, people can suffer heat stress and illness that can turn fatal even at much lower ambient temperatures.” 

In fact, say CSE researchers, a heat index of 41°C is considered dangerous to human health. According to the US National Weather Service, heat index (HI) is a measure of how hot it really feels when humidity is factored in with the actual temperature. The CSE study indicates that as is the case with RH, seasonal HI too has been on a spike since 2011. 

Days with high ambient temperatures are declining, but days with dangerously high heat index (HI) are on a rise: In 2023, the daily maximum ambient temperature for Delhi (based on the average of Palam and Safdarjung IMD stations) crossed the 40°C threshold on only 17 days. This is less than half the average of 2001-10, which stands at 46 days. 

The summers of 2020 and 2021 had also registered significantly less days with 40°C+ temperatures compared to the 2001-10 average. The highest number of days with 40°C+ ambient temperature was recorded in 2018 (55 days), followed by the summers of 2012 and 2022 (52 days each). Overall, there is a declining trend in the number of days with very high ambient temperatures, finds the CSE report.  

Says Somvanshi: “Looking only at daily maximum temperature data is not a good measure, as daily average temperature and humidity are critical parameters too for human comfort. The human body is more stressed handling humid heat than dry heat. This summer, there have been 14 days when the daily average heat index crossed the danger threshold of 41°C. The summer of 2020 had 32 days with HI exceeding 41°C -- that has been the maximum in any year for the study period.” 

Monsoon is more thermally uncomfortable than the drier pre-monsoon season: A Delhi summer can be divided into two distinct periods: pre-monsoon or dry heat period (March to May, as per the IMD) and monsoon or humid heat period (June to August). Naturally, relative humidity is much lower during the pre-monsoon period compared to the monsoon period. 

The study found that average RH has significantly increased for both pre-monsoon and monsoon periods compared to the 2001-10 average. Increasing humidity significantly alters the heat index (“what it feels like” temperature) in the monsoon period. It adds an additional 5-8°C heat to the ambient temperature. This monsoon, it added 6.7°C on an average. This was much higher than the average of 2001-10 monsoons (5.6°C). This increased thermal discomfort as traditional evaporative cooling methods like desert coolers became ineffective. 

Delhi is not cooling down at night: Hot nights are as dangerous as mid-day peak temperatures. People get little chance to recover from day-time heat if temperatures remain high overnight, exerting prolonged stress on the body. A 2022 study published in the Lancet Planetary Health by scientists from China, South Korea, Japan, Germany and the US has noted that the risk of death from excessively hot nights would increase nearly six-fold. This prediction is much higher than the mortality risk from daily average warming suggested by climate change models.

In the decade 2001-10, ambient air temperature used to come down by 14.3°C (on an average) from the day-time peak to night-time low during the pre-monsoon season. In 2023, it cooled down by only 12.3°C. 

The CSE analysis notes that there has been a lowering in Delhi’s ability to cool down at night throughout the study period starting 2018 (compared to the 2001-10). A similar phenomenon is noted in the land surface temperature (LST), which used to be -- on an average -- 15°C lower than day-time LST in 2001-10. But now it cools down by just 12.2°C as noted during the 2023 pre-monsoon season. This is a 14-19 per cent loss in cooling effect of the night on the city this year. 

The situation worsens during the monsoon season (June-August). In 2023 monsoon, ambient temperature and LST cooled down just by about 7.5°C each compared to the 2001-10 decadal average of 8.5°C and 9.6°C, respectively. It is a 12-21 per cent loss in the city´s daily self-cooling ability. 

Delhi’s core is not cooling down at night at the same rate as the peri-urban areas: Higher green cover in the city core has helped to keep it comparatively cooler during the day, but not as effectively during night. At night, the city core is 2.8-4.3°C hotter during the pre-monsoon season and 0.9-1.5°C hotter during monsoons. During day-time, the core of Delhi is 0.6-1.1°C cooler than its peripheries and peri-urban areas both during per-monsoon and monsoon seasons. The reason for this requires further assessment. 

Electricity consumption is increasing due to high heat and humidity in Delhi: Monsoon discomfort is reflected in Delhi´s growing demand for electricity. On an average, daily peak electricity demand during monsoons has been over 40 per cent higher than the peak demand during pre-monsoon season in last six years. In 2023, daily peak demand during monsoons has been 47 per cent or about 2,000 kW higher than the pre-monsoon average. In fact, the average daily peak demand this monsoon has been the highest since real-time load monitoring started in the city in 2018. 

Every degree increase in heat index leads to 140-150 MW increase in power demand in the city: Delhi’s power demand is closely linked to outdoor temperature and humidity conditions. The demand is at its minimum when the outdoor heat index is between 17.5-22.5°C (daily mean) during February and October. Power demand increases if heat index increases from that comfort spot. Increase in power demand is significantly higher when it gets hotter (compared to when it gets colder). Every degree increase in heat index beyond 22.5°C adds 140-150 MW to the city´s electricity demand. Majority of this additional demand can be attributed to space cooling. 

Delhi is consuming more electricity at night due to changing nature of heat in the city: Night-time power demand has a similar (to whole day power demand) but starker relationship with outdoor temperature and humidity conditions. Night-time demand increases by a staggering 190-200 MW for every degree increase in outdoor heat index beyond 22.5°C: almost one-third higher than the day rate. Unlike whole day demand, night-time demand does not significantly increase if the heat index drops below the threshold of 17.5°C, implying people are not using electricity to keep warm during cold nights.   

In the last six years, daily peak electricity demand was recorded at night on 194 days and 363 days during pre-monsoon and monsoon seasons, respectively. In comparison, the daily peak happened in the afternoon only on 118 days and 173 days during pre-monsoon and monsoon seasons, respectively. 

Land surface changes have a significant impact on distribution of land surface temperature: Waterbodies and green cover tend to have lower surface temperature values, whereas built-up areas, soil, and open bare lands have higher surface temperatures. Delhi shows distinct patterns of land surface temperature and heat island expansion during the years 2003, 2013, and 2012. 

Delhi is both greener and more concretised than it was two decades ago: Delhi has witnessed a significant increase in its green cover area, from 196 sq km in 2003 to 359 sq km in 2022. This points to a gradual rise in percentage share of the city’s geographical area from 32.6 per cent in 2003 to 44.2 per cent in 2022. This growing green cover includes forests, recreational parks, sparse vegetation and agricultural land, and influences the decline in LST in the respective regions. 

The central part of the city, comprising the northern regions of the south, southeast, and northeast areas of New Delhi, has a higher proportion of green cover. This area records lower surface temperature values (30°C to ∼38°C on exceptionally hot days). On the other hand, over the past three decades, Delhi has experienced significant and rapid urban expansion in all directions, especially in the east, south, southwest and southeast zones. 

The way forward

  • Delhi´s Heat Wave Action Plan is largely an emergency plan – the city needs stronger long-term solutions to mitigate heat: The city needs a Heat Resilience Plan which not just provides for emergency health services to victims of heat waves, but also helps reduce the heat in the city by eliminating waste heat accumulation. Also needed are more comprehensive long-term measures for all heat generators.
  • Heat-proof cities: There is a need to prepare inventories of waterbodies and existing and potential green areas for protection and augmentation. Any development must adhere to minimum greening requirements per hectare of development as well as vertical greening. Rationalise clustering of buildings and layout for climate proofing. Reduce traffic volume by improving accessibility, connectivity, non-motorised transport, and fleet electrification. Address location of industries and use of waste heat. Cool down concrete with shading and cool surfaces. Each strategy requires a roadmap, timeline and mandate for implementation.
  • Cool down buildings to improve adaptive thermal comfort: Implement thermal comfort requirements as per India’s Cooling Action Plan. A binding mandate is necessary for widespread adoption of passive architectural measures including day lighting, ventilation, orientation, shading, appropriate material for improving thermal comfort, and measures to reduce air conditioned hours and electricity consumption.
  • Institutionalise data analytics and science for tracking climatic and meteorological variables for heat management.
  • Improve citizen’s access to climate data, early warning systems and awareness to build support for wide ranging interventions. 

For more information, interviews etc, please contact Sukanya Nair at sukanya.nair@cseindia.org, 8816818864