In February 25th 2020, the first SARS-CoV-2 was confirmed in Barcelona, Spain. Since then, Spain has become the second country worldwide in cases (after the USA) and deaths (after Italy). Madrid and Catalonia where hit the hardest.
Many factors can affect coronaviruses' transmission, including the weather, so understanding its effect on COVID-19's transmission is essential, but evidence is lacking. An association with temperature is suggested by a study of the four major Chinese cities, and preprints indicate that low temperatures make transmission easier. These are not yet peer reviewed, though.
For the days between March 2nd and April 5th 2020, the incidence rates confirmed with PRC tests in Barcelona's Health Region and daily maximum temperatures were gathered. The methods used to study this data accounted for several factors that could cause mistakes.
On average, each increase of 1°C reduced diagnosed incidents infected that day by 7.5%%, but had little to no effect on the following days. This seems to behave linearly.
This research's accuracy is improved compared to other similar ones, because a smaller area was used, and so geographical factors aren't altering the results. On the other hand, other weather factors such as humidity and UV radiation were not taken into account. Daily deaths would be a better measure, as they are less variable on the amount of daily tests, but their distance from the day of infection would be larger and more variable.
Relatively few days were considered, so further research is needed, but this could hint at a temperature-transmission relationship, as displayed on the genetically-similar SARS.