The extent of variations
A change in UV exposure by 1 standard deviation, the analysis found, reduced the growth rate of new cases by around 1 percentage point over the following two weeks. A change of 1 percentage point, for context, is approximately equivalent to the change in UV between April and May in Delhi.
Based on changes in UV, the model predicted Covid-19 growth rates for the temperate zones north and south of the tropics. Between January and June, Covid-19 growth rates would increase by 7.3 percentage points in southern temperate locations and decline by 7.4 percentage points in the northern temperate ones. And when the seasons flip, growth rates in December compared with July were predicted to decrease by 7.7 percentage points in southern temperate regions, and jump by 7.8 percentage points in northern areas.