The Financial Times published the following chart last night. It shows the cumulative number of COVID-19 cases around the world, across the number of days since the 100th case in that particular country. The message here is that most western countries appear to be on a similar trajectory. (The grey dotted line represents a 33% daily increase.) Whereas in Asia, and in particular Hong Kong and Singapore, they have seemingly managed to slow the spread.

Now, there are a number of possible explanations for the outliers; everything from stricter quarantine rules to more rigorous testing. There's also an argument that Hong Kong and Singapore were better prepared as a result of the SARS outbreak in 2002. (More on these explanations, here.) But the other factor at play seems to be climate.

A recent study (by Jingyuan Wang, Ke Tang, Kai Feng, and Weifeng Lv) has concluded that, like the flu, the transmission of COVID-19 appears to be significantly impacted by both air temperature and relative humidity. In their research, they looked at the reproductive number (R), or the severity of infectiousness, for all Chinese cities with more than 40 cases between January 21 to 23, 2020. (Large-scale government interventions began on January 24, 2020 and would have therefore skewed the numbers.)

What they found was that for every one degree Celsius increase in temperature and every one degree Celsius increase in relative humidity, the reproductive numbers drop by 0.0383 and 0.0224, respectively. Air temperature, in other words, has more of a positive impact on containing spread than relative humidity -- which feels right. That is also apparent when you look at the above charts. Take note of Korea, Iran, and Italy near the top left corner of the temperature chart.

If you'd like to download a full copy of the research paper, click here.