But humidity turns out to be a bigger player here. It comes down to physics, much of which researchers are still trying to understand, said Spencer Fox, a data scientist who has published research on the seasonality of the flu and the epidemiology of viruses. A virus like the flu spreads when people sneeze and spray aerosolized droplets of gunk and virus into the air. Humidity affects how long those droplets hang out where other humans can breathe them in and how well the virus can survive inside the droplet.
One theory is that the low humidity in winter means the liquid coating protecting the virus droplet doesn’t evaporate as quickly, Yang said. This theory would also explain why tropical areas have a flu season associated not with cold weather but with extremely high humidity — another situation when there’s not much evaporation happening. In between those extremes, the liquid in the virus-carrying droplets tends to evaporate, concentrating solids and making an environment less hospitable to viral survival, she said.
Again, though, this is from research on the flu. It doesn’t necessarily mean the virus that causes COVID-19 would work the same way. The thing we call a “common cold” can sometimes be caused by a coronavirus, and Yang said colds aren’t as strongly seasonal as the flu is. Another counterexample, Yang said, is SARS, which in 2003 made some people sick in late spring in Toronto — in other words, a place with warmer conditions similar to those in much of the United States. (SARS was also caused by a coronavirus — there are many.) In other words, waiting for the season to change should not be your plan for stopping the spread of a novel virus.