What matter mosts, heat or humidity? The conundrum of warm versus cold weather can be untangled a bit if you consider humidity as a cofactor with heat. The short answer is that more aerosolized virus particles are in transit when it is arid and less aerosolized virus particles are in transit when it is humid. The related temperature is largely irrelevant. Humidity slows the evaporative process whereas increasing temperature accelerates it.
Why does this matter for COVID? No one breathes out the virus all by itself. We exhale water droplets of various size that are much much larger than a virus, but the evaporative process begins immediately on exit of the mouth if the local environment is arid. Keeping a virus contained in a large droplet is IDEAL because even a lousy mask will stop transmission or the droplet will fall to the ground in less than 6 feet. A good mask is IDEAL in all environments because it is capable of stopping the smaller aerosols too. BTW … a good mask is a simple as layering cotton and chiffon together, which gives a 97% filtration efficiency down to 0.3 microns. Note that an N95 is only 95% efficient for that.
Why is colder weather typically associated with an increase in virus transmission? When it is cold outside, people tend to go inside and crank up the heat, thus making evaporation happen. Unfortunately, this reduces the efficiency of your respiratory tract cilia, which takes away one of your bodies weapons in the fight against the virus. More virus particles are in transit when it is arid and less virus particles are in transit when it is humid. Period. Why?
- Big droplets fall to the ground before they can evaporate into aerosols and float around for hours.
- Big droplets get stuck in your mask if it’s on your face. Even little droplets get stuck in your mask if it’s on your face. THIS is why wearing masks and practicing social distancing WORK so well!
These two practices remove at least 90% of virus-laden particles from the air that you breath indoors, which is where essentially ALL of the transmission happens. ALL of the countries who have kicked COVID-19 to the curb had near universal adoption of masks plus social distancing AND they had extensive testing and contact tracing. The United States of America are suffering because we are doing a lousy job at all 4, especially the last two.