Understanding how environmental factors impact COVID-19 transmission informs global containment efforts. We studied the relative risk of COVID-19 due to weather and ambient air pollution. We estimated the daily reproduction number at 3,739 global locations, controlling for the delay between infection and detection, associating those with local weather conditions and ambient air pollution. Controlling for location-specific fixed effects and local policies, we found a negative relationship between the estimated reproduction number and temperatures above 25oC, a U-shaped relationship with outdoor ultraviolet exposure, and weaker positive associations with air pressure, wind speed, precipitation, diurnal temperature, SO2, and ozone. We projected the relative risk of COVID-19 transmission due to environmental factors in 1,072 global cities. Our projections suggest warmer temperature and moderate outdoor ultraviolet exposure may offer a modest reduction in transmission; however, upcoming changes in weather alone will not be enough to fully contain the transmission of COVID-19.