Published: 10 June 2026 | Author: Team JanVayu | Reading time: 5 min
On a peak summer afternoon, CSE's analysis of Landsat 8/9 satellite data for Delhi found land-surface temperatures reaching 60.77°C at places such as IGI Airport and barren or unsown land, while the area around the Yamuna hovered near 33°C. Lutyens' Delhi, Civil Lines and the Cantonment stayed below CSE's heat-stress threshold, which CSE credits to low density and tree shade. Same sun, same city. The ground differed.
This is the urban heat island effect, and we have added a panel about it to JanVayu: Urban Heat Island, under Health & Trends. A fair question came up while we built it, so here is the answer: what is heat doing on an air-quality website?
Heat and dirty air are one problem
They share chemistry, weather, causes and victims.
Heat makes ozone. Ground-level ozone is not emitted by any tailpipe or chimney. It forms in the air when nitrogen oxides and volatile organic compounds react in sunlight, and the reaction runs faster as the air gets hotter. Bloomer et al. (2009) found ozone rising roughly 3 parts per billion for every 1°C of warming across two decades of data. Atmospheric chemists call this the "climate penalty." The same traffic makes more ozone on a 50°C street than on a 35°C one.
Hot days are often still days. Still air does not carry heat away, and it does not disperse smoke, dust and exhaust either, so pollution piles up. Jacob & Winner (2009) reviewed projections that a warmer climate will be more stagnant.
Cooling feeds the problem. Hotter colonies run more air conditioners. That draws more electricity from a coal-heavy grid, which means more PM2.5 and SO₂, and every AC dumps its waste heat back onto the street. Salamanca et al. (2014) simulated a heat episode in Phoenix, USA and found that AC waste heat raised night-time 2 m air temperature by more than 1°C in some urban locations. Heat raises pollution, and pollution adds to heat.
Trees do two jobs. Tree cover lowers temperature and also traps particulate matter on its leaves (Nowak et al., 2014). Strip the green from a neighbourhood and you remove the shade and the filter together.
The victims overlap too. On a hot, polluted day, heat and dirty air strain the heart and lungs through the same pathways. A nine-city European study (Analitis et al., 2014) found heat-wave deaths were 54% higher on high-ozone days (among people aged 75–84). A 620-city, 36-country analysis (Stafoggia et al., 2023) found suggestive evidence that heat and air pollution reinforce each other. In Delhi, the Artha Global survey below found that poorer, densely built areas remain hotter because of limited access to green cover.
What the Delhi survey found
A January 2026 white paper from Artha Global, Mapping Heat Inequality Across Neighbourhoods in Delhi (Sircar et al.), surveyed 2,368 households across all 70 of Delhi's assembly constituencies. It put numbers on the picture:
- Raising built-up area from 25% to 55% added +0.6°C of experienced heat.
- Raising tree cover from just 3% to 11% removed −1°C.
So trees cool more than concrete heats. Green cover is the most effective cooling infrastructure a neighbourhood can have, and it cannot be added once a colony is fully built, because there is no room left for the canopy. The decision is made at the planning table, not afterwards.
Delhi is the example, and the physics is national
We lead with Delhi because of the CSE and Artha Global work described above. None of the chemistry is specific to Delhi, though, and the panel lets you check your own city: does the afternoon ozone peak ride the afternoon heat peak?
The panel includes a live chart that plots today's hourly ozone against hourly air temperature for any Indian city you choose (data from the free, key-less Open-Meteo API). Pick your city and the two curves climb together through the morning and crest in the afternoon. That shared peak is the climate penalty, happening today where you live.
There is also an interactive estimator. Set your neighbourhood's built-up percentage and tree cover, and it returns how much hotter or cooler you run than a typical Delhi block, using the Artha Global coefficients.
Why this belongs in the archive
JanVayu is a record of India's air crisis. Heat belongs in that record because cooling a neighbourhood and cleaning its air are increasingly the same project. Plant the canopy and you get shade and a filter. Pave it over and you lose both, and you cannot easily get them back.
Explore it: Urban Heat Island on JanVayu.
Sources: Sircar, N. et al. (2026), "Mapping Heat Inequality Across Neighbourhoods in Delhi," Artha Global; surface-temperature figures from Centre for Science and Environment, Making Delhi Heat-Resilient: A Roadmap with the Focus on Vulnerable Groups (2026); Bloomer, B.J. et al. (2009), Geophysical Research Letters 36, L09803; Jacob, D.J. & Winner, D.A. (2009), Atmospheric Environment 43(1):51–63; Analitis, A. et al. (2014), Epidemiology 25(1):15–22; Stafoggia, M. et al. (2023), Environment International 181:108258; Salamanca, F. et al. (2014), J. Geophysical Research: Atmospheres 119:5949–5965; Nowak, D.J. et al. (2014), Environmental Pollution 193:119–129. Live hourly ozone and temperature via Open-Meteo (CAMS).