COVID-19 mortality: positive correlation with cloudiness and sunlight but no correlation with latitude in Europe

Abstract

AbstractWe systematically investigated an ongoing debate about the possible correlation between SARS-CoV-2 (COVID-19) epidemiological outcomes and solar exposure in European countries, in the period of March – August 2020. For each country, we correlated its mortality data with solar insolation (watt/square metre) and objective sky cloudiness (as cloud fraction) derived from satellite weather data. We found a positive correlation between the monthly mortality rate and the overall cloudiness in that month (Pearson’s r(35)=.779, P<.001; linear model fitting the data, adjusted R2 =0.59). In Europe, in colder months, approximately 34% to 58% of the variance in COVID-19 mortality/million appears to be predicted by the cloudiness fraction of the sky, except in August in which only ∼15% of the variance was explained. The data show a low, negative correlation between the mortality rate with the overall insolation received by the country area in that entire month (Pearson’s r(35)=-0.622, P<.001). Additionally, we did not find any statistically significant correlation between the mortality and the latitude of the countries when the “latitude of a country” was precisely defined as the average landmass location (country centroid). The unexpected correlation found between cloudiness and mortality could perhaps be explained by the following: 1) heavy cloudiness is linked with colder outdoor surfaces, which might aid virus survival; 2) reduced evaporation rate; 3) moderate pollution may be linked to both cloudiness and mortality; and 4) large-scale behavioural changes due to cloudiness (which perhaps drives people to spend more time indoors and thus facilitates indoor contamination).