US government researchers have determined that the virus survives best indoors and in dry conditions, and loses potency when temperatures and humidity rise - and especially when it is exposed to sunlight, said William Bryan, acting head of the U.S. Department of Homeland Security's Science and Technology Directorate.
"The virus dies quickest in the presence of direct sunlight," he told a White House news briefing.
The findings could bolster hopes that the coronavirus will mimic the behavior of other respiratory diseases like influenza, which typically are less contagious in warm weather.
But the coronavirus has also proven lethal in warm-weather places like Singapore, raising broader questions about the impact of environmental factors.
President Donald Trump said the findings should be interpreted cautiously. "I hope people enjoy the sun and if it has an impact, that's great," he said.
On nonporous surfaces like stainless steel, the virus takes 18 hours to lose half its strength in a dark, low-humidity environment, Bryan said.
In a high-humidity environment, that half-life dropped to six hours, and when the virus was exposed to high humidity and sunlight, the half-life dropped to two minutes, he said.
Researchers found a similar effect with the coronavirus that was suspended in the air - simulating the coughing or sneezing that often spreads the disease. In a dark room, the virus maintained half its strength for an hour. But when exposed to sunlight, it lost half its strength in 90 seconds, Bryan said.