Sweltering Nights: Decoding Urban Heat Stress in Delhi

FOCUS

This case study on the analysis of heat stress in Delhi was written by Avikal Somvanshi and Sharanjeet Kaur. It was published by the Centre for Science and Environment, a New Delhi-based public interest research and advocacy organization, on October 26, 2023. 

Recent scientific studies have shown that rising temperatures have serious adverse impacts on urban areas and that there is a need for city-specific heat wave management plans. However, most existing emergency action plans do not consider several factors that affect temperatures in urban areas, the study notes.

This report looks into weather conditions in summer (March to August) which are impacted by not just daytime temperatures, but also nighttime temperatures, humidity, surface heat absorption, land surface temperature, changing land-use and electricity consumption. The research is based on a statistical analysis of temperature and humidity levels in Delhi since 2001. On the basis of this analysis, the study recommends having a heat action plan for Delhi that focuses on reducing the accumulation of waste heat, that is, heat produced as a byproduct of machines at work. 

The 36-page study contains three sections: Introduction (Section 1); Key Findings (Section 2); and Way Forward (Section 3).

    FACTOIDS

  1. The report states that though the seasonal ambient temperature in Delhi has been stable since 2011, the city is becoming more humid. This is the effect of unseasonal summer rains.

  2. Average relative humidity (RH) recorded in the last decade has been 14 per cent higher than the 2001-10 average. Due to this, the heat index (HI) has also been on the rise since 2011 – heat index crossing 41°C is considered dangerous to human beings. Summer 2023 recorded 14 days when the daily average heat index crossed that mark.

  3. Monsoons in Delhi have become more uncomfortable than pre-monsoons. This is evident through the fact that compared to 2001-2010, there is a 19 per cent increase in the number of days when the HI crossed the 41°C mark in monsoons during 2011-2023. However, there are almost no such days recorded in pre-monsoons.

  4. Heat and humidity have driven up consumption of electricity in Delhi, the report notes. Average daily demand for electricity in 2023 peaked during monsoon – 47 per cent or 2,000 kilowatts more than during the pre-monsoon season.

  5. Nights in Delhi have started getting hotter, with a 14-19 per cent loss of cooling effect at night. During 2001-2010, land surface temperature (LST) at night used be an average of 15°C lower than daytime LST. In 2023, the difference had narrowed to 12.2°C in the pre-monsoon season.

  6. Delhi has seen rapid urban expansion which has led to a rise in land surface temperature inside the city. Between 2003-2022, the total heat-absorbing built-up area rose from 467.8 square kilometres to 568.9 square kilometres. In 2022, LST in the city was recorded to range from 38°C to 40°C when earlier it was 30-33°C.

  7. There has been a rise in green cover in the inner city, but it has only helped cool the city during the day and not at night. In pre-monsoon, the inner city is two per cent cooler than the periurban during daytime but 23 per cent hotter at night. In monsoon, the inner city is three per cent cooler than periurban during daytime but six per cent hotter at night.


    Focus and Factoids by Radhika Gupta.

AUTHOR

Avikal Somvanshi and Sharanjeet Kaur

COPYRIGHT

Centre for Science and Environment, New Delhi

PUBLICATION DATE

26 Oct, 2023

TAGS

#environment #climate-change #delhi #heat-stress

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