This paper investigates how air temperature and humidity influence the transmission of COVID-19. After estimating the serial interval of COVID-19 from 105  pairs of the virus carrier and the infected, 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 for 100 Chinese cities , high temperature and high relative humidity significantly reduce the transmission of COVID-19, respectively , even after controlling for population density and GDP per capita of cities. One degree Celsius increase in temperature and one percent increase in relative humidity lower R by 0.0383 and 0.0224 , respectively. This result is consistent with the fact that the high temperature and high humidity significantly reduce the transmission of influenza  . It indicates that the arrival of summer and rainy season in the northern hemisphere can effectively reduce the transmission of the COVID-19. 
This paper investigates the influence of air temperature and relative humidity on the transmission of COVID-19. After estimating the serial interval of COVID-19 from 105 hand-collected pairs of the virus carrier and the infected, 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. This result is consistent with the fact that the high temperature and high humidity reduce the transmission of influenza and SARS . It indicates that the arrival of summer and rainy season in the northern hemisphere can effectively reduce the transmission of the COVID-19. We also developed a website to provide R of major cities around the world according to their daily temperature and relative humidity: URL
R
how
the influence of
R
humidity influence
relative humidity on
A
hand-collected
D
for 100 Chinese cities
D
relative
R
, even after controlling for population density and GDP per capita of cities. One degree
. One-degree
R
0.0383 and 0.0224
0.0225 and 0.0158
D
significantly
A
and SARS
A
We also developed a website to provide R of major cities around the world according to their daily temperature and relative humidity: URL