Last April, as people around the world sheltered in place against the Covid-19 pandemic, The Indian Express newspaper published a photo that had gone viral on Twitter, showing a slightly hazy deep blue sky over Uttar Pradesh, the most populous state in northern India. Above a garden trellis, the angular, white peaks of the Himalayan mountains were visible on the horizon like stiff whipped meringue. Pawan Gupta, a senior scientist with the Universities Space Research Association at NASA Marshall Space Flight Center, says that friends and family in India told him the peaks had not been that visible for decades. The reason why is simple: Before the pandemic lockdowns, the air was filled with smog.