Despite multiple studies, there is currently limited agreement on the impact of weather conditions on transmission rates of COVID-19.
We assembled one of the largest datasets of COVID-19 infection and weather and analyzed the impact of weather on the transmission of the virus across 3,739 global locations. For details on the methodology, data, and estimated statistical models, see our working paper.
We have developed interactive figures to visualize evidence-based projections of the impact of weather on the potential transmission rate of COVID-19 from May 1, 2020 to April 30, 2021, using 2019-2020 weather data for each location.
See also interactive visualizations based on our results in The New York Times.
Projecting How Weather and Air Pollutants Impact Transmission Rate
Reproductive number, the average number of individuals infected by each infectious person, is key to understanding transmission of diseases. Absent prior immunity and policy responses, this number is estimated to be in the range of 2-3 for COVID-19 across many locations, leading to rapid growth in the number of cases. Figures below show how that number is impacted by weather and air pollutants in each location, captured in the Relative COVID-19 Risk due to Weather and Air Pollution (CRW). CRW compares the relative changes in reproductive number for the disease due to weather factors (average and diurnal temperature, ultraviolet (UV) index, humidity, pressure, precipitation) and air pollutants (SO2 and Ozone). For example, a shift over a season in CRW from 1 (the 95 percentile in our sample) to 0.7 in a given location points to a 30% reduction in estimated reproduction number over that period due to weather and air pollutants (i.e. assuming everything else is constant).
We project the CRW for the next year across the world using weather data (averaged over a moving window of 15 days) from 2019 period. Our projections suggest warmer times of the year, and locations, may offer a modest reduction in reproductive number, helping with efforts to contain the pandemic and build response capacity. Outdoor UV is also associated with lower transmission up to a point (index of ~6.5), but above the effect reverses, creating an overall U-shaped relationship. Pressure, precipitation, diurnal temperature, lack of humidity, SO2 and Ozone all modestly contribute to increased transmission rates. Overall, CRW numbers rarely go below 0.5 or abouve 1.5, indicating that upcoming changes in weather alone will not be enough to fully contain the transmission of COVID-19.
Our projections suggest warmer temperature and moderate outdoor UV exposure may offer a modest reduction in reproductive number; however, upcoming changes in weather alone will NOT be enough to fully contain the transmission of COVID-19.
Important Note: CRW numbers do NOT reflect the total risks of COVID-19 across different locations, rather, only the portion of the risk due to weather and air pollutants. Beyond weather, those risks depend on population density, cultural practices, individual and policy responses, and a host of other factors not included in our projections (but controlled for in our estimation using fixed effects and location-specific trends). Therefore, comparisons between locations are only informative about the relative impact of weather on transmission, keeping everything else constant.
(Projection tools are best viewed on desktop or tablet)
Ask questions about this research
- The new version of our working paper on the impact of #Weather on #COVID19, including new validation tests and air pollution variables, is now available online: t.co/UEfKM0YMCz Thanks again to everyone who provided feedback and suggestions. (PS: We have a new title!)
- @davidebradford @Harvard @harvardmed @MGH_RI @mitsloanexperts @mgh_ita @virginia_tech @UConn Thanks David for sharing your thought. We are aware of such uncertainty; indeed, we have considered it in our project. We discuss how our analysis is fairly robust to those errors/underestimations. Check out the next version of our paper for details; should be online in 1-2 days.
- There is still so much to learn about COVID-19. This was sent to me, and on reading, what struck me is how complex studying some of these relationships can be, especially while the spreading of the virus is still underway. t.co/xKiB70Rxx3
- The New York Times released a nice coverage of our research on the impact of #weather on #COVID19 with great interactive visualizations. Kudos to amazing colleagues who have been working tirelessly on this important work t.co/OY4a1hlwih @MassGeneralNews @harvardmed
- Our team has been working hard to test and wrap up new analyses in our #weather & #COVID19 research, including air pollution and ultraviolet (UV) exposure. New results should be online asap this week, stay tuned! Thanks to everyone who provided feedback over the last two weeks
- Many folks are spreading some absolute nonsense about 77F being a magic number in terms of COVID19 transmission. Here's the actual scientist trying to correct the media's absolute failure to report the findings correctly: t.co/IYAALxNra3