2  Data and methods
In this study, global COVID-19 case datasets were downloaded from the Center for Systems Science and Engineering at Johns Hopkins University. Data of more than 3,750,000 confirmed COVID-19 cases from 185 countries/regions from January 21, 2020 to May 6, 2020 were included in this paper. The daily numbers of COVID-19 cases are being reported for each province or state in several large countries such as China and the United States. Zonal mean COVID-19 cases on each day were calculated and fitted using normal distribution. The center and the standard deviation of the fitted curve were used to show the most serious dispersal regions.
Corresponding observational data of daily air temperature and dew-point temperature were obtained from the Weather Underground website (https://www.wunderground.com/). The relative humidity and the absolute humidity were calculated using daily air temperature and dew-point temperature. Maximum/minimum temperatures and temperature difference on each day were also calculated for each state/country. Furthermore, time-series of zonal mean temperature at 1000 mb in 2019 was calculated from National Centers for Environmental Prediction reanalysis data to study the trend of approximate global COVID-19 dispersal.
In addition, the Gridded Population of the World from 1980 to 2010 was developed by the Center for Global Environmental Research at the National Institute for Environmental Studies, Japan (Murakami and Yamagata, 2019). We calculated zonal means of gridded populations to show the locations with high population density.