Before revision:  we calculate the daily effective reproductive number, R, for each of all 100 Chinese cities with more than 40 cases . Using the daily R values from January 21 to 23,2020 as proxies of non-intervened transmission intensity, we find, under a linear regression framework, high temperature and high humidity significantly reduce the transmission of COVID-19 , respectively.One-degree Celsius increase in temperature and one percent increase in relative humidity lower R by 0.0225 and 0.0158,respectively.
After revision:  we calculate the daily effective reproductive number, R, for each of all 100 Chinese cities with more than 40 cases . Using the daily R values from January 21 to 23,2020 as proxies of non-intervened transmission intensity, we find, under a linear regression framework, high temperature and high humidity significantly reduce the transmission of COVID-19 for both China and the U.S. before lockdown in both countries: one-degree Celsius increase in temperature and one percent increase in relative humidity lower R by 0.0225 and 0.0158,respectively.