Pollution Led Decline in Sunshine Hours May Hit Solar Power Output in India

Sunshine hours across India have been steadily declining over the past three decades, a trend linked to thicker cloud cover and increasing aerosol pollution, according to a study published in Nature’s Scientific Reports.

Conducted by scientists from Banaras Hindu University, the Indian Institute of Tropical Meteorology, and the India Meteorological Department, the study found significant annual drops in sunshine across most regions, particularly the north Indian plains. The study attributed this trend to increased cloud cover and rising aerosol pollution.

The steepest drop was reported in the north Indian plains, losing an average of 13.1 hours per year, while the west coast saw a loss of 8.6 hours annually.

The study analysed data from 20 meteorological stations across nine regions between 1988 and 2018 to arrive its conclusions. In addition, the report also stated that annual sunshine hours, which refers to the time when sunlight is bright enough to be recorded as a sunshine hour, have decreased in all locations except the northeast, where a marginal seasonal ‘levelling off’ was observed.

Sunshine hours decreased sharply during the monsoon months (June to September) and increased during the drier months (October to May), the study revealed.

Further, BHU scientist Manoj K Srivastava told TOI that the west coast recorded the sunshine hours fall by 8.6 hours per year, while the north Indian plains recorded the steepest drop at 13.1 hours a year on an average. The east coast and Deccan plateau also indicated declining trends of 4.9 and 3.1 hours per year, respectively. Meanwhile, the central inland region showed a loss of about 4.7 hours per year.

The report attributed the long-term ‘solar dimming’ to an increase in aerosol concentrations, the tiny particles that get released from industrial emissions, biomass burning and vehicular pollution.

These aerosols act as seeds for water vapour to condense and form clouds. When there are too many aerosols, they create many smaller and longer-lived cloud droplets instead of fewer large ones. These small droplets don’t fall as rain easily, so the clouds stay in the sky longer and block sunlight for extended periods—making the sky look overcast.

“This year's monsoon, too, has been marked by persistently cloudy skies across much of India, especially over the west coast, central India and the Deccan plateau, which saw frequent overcast conditions even on days without rain. Though the study period spans till 2018, trends remain just as relevant today, as the same haze, humidity and cloud patterns persist - if anything, more strongly than before," a scientist told TOI.

Impact on Solar Forecasts

India's plans for solar energy generation could be at risk due to this decrease in sunshine.

Forecasts of solar yield must now be modified appropriately by investors and planners. According to a 2024 study published in the journal Climate, aerosols and clouds reduce surface solar radiation (SSR), the amount of solar energy that reaches the Earth's surface, by up to 13.33% on average, with clouds further blocking the skies. Planners must adjust forecasts for solar yield accordingly.