Is COVID-19 growing in Europe and North America because it is cold there?

The field is heavily divided on this. While some people are speculating about the fact that a relatively warmer weather might slow down the spread of COVID-19 infection and the viral ‘titer’ around us, others think that it would be highly unlikely that Europe and North America are worst hit due to the prevalent ‘cold weather’ there. Let me start with the latter.

Firstly, both Europe and North America are continents that spread across the temperature zones. Both continents have parts which spread across the tropical, temperate and arctic regions. See the maps, for example:

(from dailyhive.com)

(from wikipedia)

If the relationship between viral progression and temperature was true, we would have expected to see a roughly similar pattern of temperature and COVID-19 infections. Looking at the maps, it is clear that it isn’t true (look at tropical countries like Indonesia, Malaysia, etc. which have a heavy infection rate). The fact that North America and Europe have a heavy infection rate can be attributed to multiple factors, which are being currently probed on by epidemiologists. 

There is, however, a contrasting meta-analytical study that both shows some interesting data and makes some mechanistic speculations. Quoting from a report on the study,

“Qasim Bukhari and Yusuf Jameel, both from the Massachusetts Institute of Technology, analyzed global cases of the disease caused by the virus, COVID-19, and found that 90% of the infections occurred in areas that are between 37.4 and 62.6 degrees Fahrenheit (3 to 17 degrees Celsius) and with an absolute humidity of 4 to 9 grams per cubic meter (g/m3). (Absolute humidity is defined by how much moisture is in the air, regardless of temperature.)"

In countries with an average temperature greater than 64.4 F (18 C) and an absolute humidity greater than 9 g/m3, the number of COVID-19 cases is less than 6% of the global cases.”(https://www.livescience.com/warmer-weather-slow-coronavirus-spread.html) Note here that rather than temperature, the factor at hand seems to be humidity. The researchers speculate that this might have something to do with the ‘moisture-coat’ that forms around the virus when one coughs or sneezes it out. During winters, the less humidity around makes the coat evaporate faster, which eventually leads to the virus being pulled faster to the ground by gravity. This significantly reduces the rate of infections. Conversely, during summers, the increased humidity around leads to more sustenance of virus in air, and therefore increases the rate of infection. 

With all these speculations coming in, we need to be careful of coming to a conclusion right away!