It turns out blood pressure has a chill factor: Hypertension is harder to control in colder weather, heart experts say.

But the finding, expected to be presented Monday at the American Heart Association annual meeting, in Orlando, Fla., is not exactly new.

"It has been noted for decades that people's blood pressure tends to be a little bit harder to control or a little bit higher in cold climates," said Dr. Kenneth Baker, M.D., a professor of internal medicine at the Texas A&M Health Science Center College of Medicine.

He was not involved in the study, which was led by Dr. Ross Fletcher, of the Department of Veterans' Affairs and Georgetown University in Washington, D.C.

The study drew on a vast database: the VA's storehouse of 1.8 billion vital statistics records. The five-year study looked at electronic health records for almost 1.2 million patients cared for at 15 VA hospitals located at different latitudes: Anchorage, Alaska; Baltimore; Boston; Chicago; Fargo, N.D.; Honolulu. Hawaii; Houston; West Los Angeles, Calif.; Miami; Minneapolis; New York City; Philadelphia; San Juan, Puerto Rico; Portland, Ore; and Washington, D.C.

The average age of participants was 66. Fifty-one percent were Caucasian, 21 percent were Hispanic, and 27 percent were black. Less than 4 percent were female.

Within the sample, almost 444,000 veterans had high blood pressure (based on readings of more than 140/90 on three separate days).

And, regardless of their locale, patients experienced an average difference of almost 8 percent in getting their high blood pressure back to normal between winter and summer, with that feat being much tougher in winter.

Why?

Baker posited any number of reasons.

"My guess is that one of the top reasons is, when you're in a cold atmosphere, you vasoconstrict [blood vessels narrow]," he said. "If you stick your hand in ice water, it has the same effect. Blood pressure goes up a little, and in hotter climates, sitting by the pool in your swimming suit, the vessels in the skin are dilating, you lose water and sweat off salt, and blood pressure drops."

There are other possible factors as well. Cold medicines people take in the winter can raise blood pressure, as can non-steroidal anti-inflammatory painkillers.

People also tend to be more depressed in the darker months, leading to more alcohol and coffee consumption, both of which can raise blood pressure, the expert said.

A more obvious reason: People are also often more sedentary in the winter, staying inside and eating more. This can have a secondary effect -- weight gain -- which also contributes to hypertension.

Baker believes that these biological or lifestyle differences are more likely to explain the findings than southern or northern climate or the amount of light.

Overall, however, VA hospitals in all the cities studied showed improvements of about 4 percent per year in their ability to keep patients' average blood pressure under control, the study found.