WASHINGTON - A new study on survival of coronavirus in different cities and on different surfaces shows that temperature, humidity and surface affect drying time of COVID-19 virus respiratory droplets.

In the study published Tuesday in Physics of Fluids, researchers examined the drying time of respiratory droplets from COVID-19-infected subjects on various surfaces in six cities around the world, which are New York, Chicago, Los Angeles, Miami, Sydney and Singapore.

These droplets are expelled from the mouth or nose when someone with COVID-19 coughs, sneezes or even speaks moistly. The droplet size is on the order of human hair width, and the researchers examined frequently touched surfaces, such as door handles and smartphone touchscreens.

Using a mathematical model established in the field of interface science, the drying time calculations showed ambient temperature, type of surface and relative humidity play critical roles.

Higher ambient temperature helped to dry out the droplet faster and drastically reduced the chances of virus survival. In places with greater humidity, the droplet stayed on surfaces longer, and the virus survival chances improved, according to the study.

The researchers determined the droplet drying time in different outdoor weather conditions and examined if this data connected to the growth rate of the COVID-19 pandemic.

"Understanding virus survival in a drying droplet could be helpful for other transmissible diseases that spread through respiratory droplets, such as influenza A," said Amit Agrawal, one of the authors.

The study suggests that surfaces, such as smartphone screens, cotton and wood, should be cleaned more often than glass and steel surfaces, because the latter surfaces are relatively hydrophilic, and the droplets evaporate faster on these surfaces.