While there were some reductions in rates of infection during the warm weather months, it is now known these reductions were tied to better ventilation in outdoor exposure, with lower viral loads in the air outside than in indoor spaces. The scientists combined temperature and humidity into what is called an equivalent air temperature between March and July. They used cellphone data to track human movement across U.S. states, counties, regions, international regions and the globe in combination with examination of equivalent air temperature measurements.

The researchers concluded weathers effects on viral contagion came out last on a list of factors studied. While weather surely affects human behavior, the primary driving force in contagion, these are the rankings of influence in transmission as told to Science News: In contrast, the data showed the clear influence of human behavior and the outsized influence of individual behaviors. Taking trips and spending time away from home were the top two contributing factors to Covid-19 growth, with a relative importance of about 34% and 26% respectively. The next two important factors were population and urban density, with a relative importance of about 23% and 13% respectively.

Earlier speculation on warmer temperatures, higher humidity and greater exposure to the suns ultraviolet radiation possibly bringing down viral loads has not survived scientific scrutiny in terms of the virus itself.

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Don Paul: Weather has less effect on Covid-19 than first thought - Buffalo News