Akin to respiratory tract infection diseases, climatic conditions may significantly influence the COVID-19 epidemic. Since the beginning of the COVID-19 pandemic, significant efforts have been made to explore the relationship between climatic condition and growth in number of COVID-19 cases. Contentious findings of either positive, negative, or no association with climatic conditions have been reported in many studies based on some early data on COVID-19 cases over a shorter time span. We integrate COVID-19 datasets with long meteorological time series of 29 countries to explore cross-country variation in COVID-19 cases and death rates with respect to temperature and relative humidity. Our empirical study reveals that temperature and relative humidity jointly influence the growth of COVID-19 cases and death rates. We generate predictive scenarios for changes in daily cases and death rates under different combinations of temperature and relative humidity. Low temperature with low humidity in a temperate climate and high temperature with high humidity in a hot and humid climate are found to surge the growth of COVID-19 cases and death rates. These relationships and our predictive scenarios can be applied to generate early warning for any future outbreak to adopt stringency policies, kick-start economic activities, prepare healthcare service plans, and target vaccination coverage.