SYDNEY — As countries in the Northern Hemisphere tilt into summer and emerge from months-long coronavirus shutdowns, winter arrives this month in subtropical parts of the Southern Hemisphere — and with it increased concern for the virus's spread.

Studies of how the novel coronavirus behaves from season to season are still in their early stages. Preliminary results show that temperature and other climatic factors have less impact on its spread than social behavior and the accompanying restrictions that governments put in place. But researchers in Australia, southern Africa and South America say colder weather is cause for significant concern.

“We need to be thinking if it’s wintertime, it could be covid-19 time,” said Michael Ward, an epidemiologist in the University of Sydney’s School of Veterinary Science, in a statement. Ward and colleagues published a paper Tuesday that found that every 1 percent decrease in humidity increased coronavirus cases by 6 percent. Australia’s winters are generally less humid than its summers, though the degree depends on the region.

Humidity is just one of many seasonal climatic, biological and behavioral variations that researchers have been investigating.

Humidity interacts with temperature as well as ultraviolet rays, for instance, and heat and UV exposure are also postulated in some ongoing studies as potential suppressors of the virus’s spread. Others are examining how those climatic factors may be confounding variables, with altitude possibly being a more important measure.

Parts of South America in the Southern Hemisphere tend to be less humid during their upcoming winter months, which has researchers in Brazil, the world’s biggest coronavirus hot spot, worried. While much of the country is tropical, its southern plains dry out during winter, and temperatures can drop below freezing.

“The United States was lucky in that it started amassing cases at the end of its winter, and the start of spring,” said Mauro Sanchez, an epidemiologist at the University of Brasília. “Our cases are growing as winter approaches. The timing was not ideal.”

Temperaturewise, the coronavirus “seems to be spread at maximum rates when daily mean temperatures are in the 5 to 11°C range [41 to 52 degrees Fahrenheit], and rate decreases steadily as the temperature rises,” according to a statement by South Africa’s government-funded Applied Center for Climate and Earth Systems Science.

Those climatic factors affect how our bodies function and how susceptible they may be. For instance, Bob Scholes, professor of systems ecology at the Global Change Institute of the University of the Witwatersrand in Johannesburg, said that in cold weather, mucus tends to form at the tip of the nose, which may promote inhalation of infected droplets, as it does with influenza and the common cold.

Then again, cold but humid weather may mean that vector droplets containing the coronavirus are larger and heavier, and therefore more likely to fall to the ground than circulate in the air and be breathed in.

“The current feeling, based on quite a lot of data, is that there is a seasonal effect, but it is small relative to behavioral changes, and so it tends to get lost in the noise,” Scholes said. “The other differences matter more.”

Winter has its own social effects and primary among them is a greater amount of time spent indoors in unventilated environments.

In South Africa, where millions rely on wood and coal stoves to cook and heat their homes, those indoor spaces will also be filled with particulate matter that some scientists believe also furthers the spread of the virus by causing coughing.

“In the winter, the use of coal and wood will be at its peak,” said Angela Mathee, director of environmental health studies at the South African Medical Research Council. “Is it a perfect storm or is it speculation? We’re in our first season, and we know very little.”

Assumptions carried over from studies of influenza that show unquestionable seasonality are tempered with skepticism rooted in the newness of covid-19, as the illness caused by the novel coronavirus is known.

Australian Nobel laureate and immunologist Peter Doherty said he was not yet convinced by the theory that the coronavirus will spread faster in the Southern Hemisphere winter.

“I have been looking and asking people and there may be an effect, but we think it’s not very big, but nobody really knows,” he said.

Scholes, too, said that while seasonality “does seem to be important,” much of the alarm raised by the scientific community has been based on educated guesses.

“We’re still at the discovery stage; we’re still looking for correlations,” he said. “To really understand climate’s effects, you need a couple years to be able to compare without making so many unproven assumptions. More smoke than light at this point, really.”

Both Australia and South Africa implemented strict lockdowns and have been aggressively testing, though they have tested only 6 percent and 1 percent of their populations, respectively.

Australia’s coronavirus death toll stands at just over 100, and 23 cases are currently being treated in hospitals. South Africa has struggled to stamp out hot spots of transmission, especially in a cluster of working-class neighborhoods in Cape Town where thousands of cases have been identified over the past week. South Africa’s death toll is just over 900, though it has a population more than double Australia’s.

Peter Collignon, a microbiologist at the Australian National University, said he expects the number of detected infections to rise from the current rate of around 100 a week to 200 or even 300 as the weather turns colder.

“The only good news for Australia is we have lots of testing and very low community transmission,” he said. But he, like others, cautioned that much of his predictions were based on experience with influenza.

Mathee, at the South African Medical Research Council, said seeing covid-19 and influenza as distinct was important for research, but that their separateness presented its own danger.

“What if you get both simultaneously?” she asked. “That’s another of the big unknowns. We could see hospitals packed with flu and covid patients, or maybe even people suffering from them as a double disease.”

Bearak reported from Nairobi. Marina Lopes in São Paulo, Brazil, contributed to this report.