Climate change has led to shift in the rainfall pattern resulting in a rise in Delhi's relative humidity, increasing discomfort even though there has been no significant change in the city's ambient temperatures since 2011, according to a new analysis.

The analysis by the independent think tank Centre for Science and Environment (CSE) underscores that the rise in heat index, due to the increase in Delhi's relative humidity, is driving the power demand in the city.

The heat index, also known as the 'apparent temperature' or 'feels-like temperature', is a measure of how hot it feels to the human body when relative humidity is factored in with the actual air temperature.

The combination of high heat and humidity can compromise the human body's main cooling mechanism -- sweating.  Evaporation of sweat from the skin cools our bodies. However, higher humidity levels limit this natural cooling mechanism.

As a result, people can suffer heat stress and illness, and the consequences can even be fatal even at much lower ambient temperatures. A heat index of 41 degrees Celsius is considered dangerous to human health.

CSE researchers point out that these local trends are consistent with the observations made by the Intergovernmental Panel on Climate Change (IPCC), a global body of leading climate scientists.

The IPCC's Sixth Assessment Report (AR6 WG-I) report says hot extremes, including heatwaves, have intensified in urban centres that are experiencing air temperatures several degrees warmer than the surrounding areas, especially during the night.

The urban heat island effect can add two degrees Celsius to local warming, reducing the adaptive capacity of cities and increasing risks, the IPCC warns.

 "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, electricity demand -- 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," says Anumita Roychowdhury, executive director, research and advocacy, CSE.

The report said the ambient average temperature for the last 10 summers is marginally lower than the decadal average for 2001-10, but this does not mean Delhi is getting cooler.

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 even though there have been no significant changes in ambient temperatures, it said.

During the pre-monsoon season in 2023, the average relative humidity stood at 49.1 per cent, which was about 21 per cent more humid than the 2001-10 average.

During the monsoon season (June to September), the average relative humidity was 73.2 per cent, 14 per cent higher than the 2001-10 average, the CSE analysis reveals.

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 degrees Celsius (daily mean) during February and October