As the number of confirmed cases of Coronavirus disease 2019 (COVID-19) continues to increase, there has been a rising concern regarding the effect of weather conditions, especially over the upcoming summer, on the transmission of this disease. In this study, we assess the transmission of COVID-19 under different weather conditions by investigating the propagation of infectious respiratory droplets. A comprehensive mathematical model is established to explore their evaporation, heat transfer and kinematics under different temperature, humidity and ventilation conditions. The transmitting pathway of COVID-19 through respiratory droplets is divided into short-range droplet contacts and long-range aerosol exposure. We show that the effect of weather conditions is not monotonic: low temperature and high humidity facilitate droplet contact transmission, while high temperature and low humidity promote the formation of aerosol particles and accumulation of particles with a diameter of 2.5 m or less (PM2.5). Our model suggests that the 6 ft of social distance recommended by the Center for Disease Control and Prevention (CDC) may be insufficient in certain environmental conditions, as the droplet spreading distance can be as long as 6 m (19.7 ft) in cold and humid weather. The results of this study suggest that the current pandemic may not ebb in the summer of the northern hemisphere without proper intervention, as there is an increasing chance of aerosol transmission. We also emphasize that the meticulous design of building ventilation systems is critical in containing both the droplet contact infections and aerosol exposures.
COVID-19: Effects of weather conditions on the propagation of respiratory droplets