At this stage we don’t really know if or how temperature affects COVID-19 transmission. But it looks like one aspect of the weather — humidity — does play a role.

We know some influenzas are seasonal, and the common cold is more common in winter. But what about COVID-19? Many people have been wondering whether the weather plays a role in its spread.

In a new study  we found the number of locally acquired COVID-19 cases in the Sydney area increased as the air became drier.

This adds to a growing body of evidence that identifies a link between humidity and COVID-19.

We found we needed only relative humidity to predict COVID-19 cases.

This relationship was consistent: we found it in both the exponential stage of the epidemic (when the numbers of cases were growing rapidly, in February and March) and when the epidemic was declining (in April and May). And the relationship was evident across different areas of greater Sydney.

We didn’t find a relationship between COVID-19 cases and temperature, rainfall or wind speed.

Our research originally began with a study in China, very early in the COVID-19 pandemic (December to February). In that study we found both drier air and lower temperatures were linked to more reported cases of COVID-19.

Coronaviruses can survive for a period of time on surfaces, and in the air. When an infected person coughs, sneezes or even talks, they can produce infectious droplets and aerosols.

By virtue of their larger size (and therefore weight), droplets land on surfaces relatively quickly.

However, aerosols are much smaller, so they persist in the air and they hang around for longer in drier air. So it follows transmission of COVID-19 is more likely when humidity is lower.