Cases of flu are on the rise, according to a recent statement from the United States Centers for Disease Control and Prevention, and experts are warning that this year’s flu season will be worse than last. Now, a new study published in the Journal of Clinical Virology is shedding some light on exactly how cold weather and the spread of viruses are linked.

It turns out, seasonal flu outbreaks first appear each year about a week after the winter’s first cold spell—or at least that’s what happened in Sweden, over the course of three years when researchers tracked weather patterns and the prevalence of the virus.

During that time, researchers collected more than 20,000 nasal swabs from people seeking medical care in and around the city of Gothenburg, and analyzed them for influenza A and other respiratory viruses. Then they compared those findings with weather data from the Swedish Meteorological and Hydrological Institute.

A surprisingly consistent pattern emerged: Each year, the first really cold week—with low humidity and temperatures below freezing—seemed to trigger the spread of flu.

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“We believe that this sudden drop in temperature contributes to ‘kickstart’ the epidemic,” said lead author Nicklas Sundell, a researcher at Sahlgrenska Academy and infectious diseases specialist at Sahlgrenska University Hospital, in a press release. “Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected.”

Airborne particles containing liquid and virus—from a sneeze, for example—can spread more easily in cold and dry weather, say the study authors. Dry air absorbs moisture from the particles, shrinking them and helping them stay in the air longer and travel longer distances.

The study found that some other common respiratory infections, such as respiratory syncytial virus and coronavirus, followed similar, temperature-driven patterns. But others, like rhinovirus (one cause of the common cold), did not seem to be affected by the weather or season.

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Sundell said that better knowledge of outbreaks based on weather may help doctors and public health experts know what’s coming—and communicate those risks to the public.

“If you can predict the start of the annual epidemics of the flu and other respiratory viruses, you can use this knowledge to promote campaigns for the flu vaccine,” he said. Hospital emergency departments could also prepare in advance, he added, for increased numbers of sick patients.

Of course, cold weather isn’t the only prerequisite for flu epidemics to take off. “The virus [also] has to be present among the population, and there have to be enough people susceptible to the infection,” Sundell explained.

And the flu obviously still spreads in climates that are warm year-round, says Nirav Patel, MD, assistant professor of infectious diseases at Saint Louis University, who was not involved in the new study.

“Clearly then, the temperature drop is important, but perhaps not the only factor,” Dr. Patel told Health via email.

He also points out that the study was only able to show a correlation between weather and flu timing in one particular region. “We’d need to see this replicated in other climate areas to assess whether this is a consistent phenomenon or is unique to influenza in Sweden,” he wrote.

Still, he says the findings are “definitely intriguing, and should be explored further.”

In the meantime, he’ll continue recommending the same things he’s always recommended for preventing the spread of flu—regardless of the temperature outside: covering your mouth when coughing or sneezing, washing your hands frequently, and getting an annual flu shot.

This article originally appeared on Health.com.