There has been much discussion on whether climate factors influence the temporal dynamics of Covid19. In this paper by Xavier Rodó et al., we provide evidence for a role of temperature and humidity. Seasonality of wintertime respiratory infections in temperate regions and airborne transmission can be seen as two sides of the same coin: winter circulation in those regions coincides with lower temperatures and lower absolute humidity, a feature that relates to smaller aerosol sizes and, therefore, facilitates persistence and enhances short-distance viral propagation.