Amidst almost unstoppable contagion, many have hung their hopes on heat and humidity as a potential defence against contracting Covid-19. In the early months of the pandemic studies of SARS-CoV-2 suggested that the virus is transmitted less efficiently in higher temperatures or at higher rates of humidity, leading to encouraging newspaper headlines around the world, from London to Jakarta. ‘Everybody hopes for seasonality,’ one US epidemiologist told the New York Times in May 2020, even as comparative reviews of research concluded that summer temperatures might slow but would not halt the transmission of the coronavirus. Against the backdrop of rising global temperatures, however, the relationship between heat and contagion demands closer scrutiny.