Cold weather, high humidity and big daily temperature swings seem to land more people in the hospital with strokes. As it got warmer, risk fell - 3 percent for every 5 degrees, the study found.
"Maybe some of these meteorological factors serve as a trigger," said Judith Lichtman, a Yale University stroke researcher who led the study. With global climate change and extreme weather like this week's freak storm in the South, "this could be increasingly important," she said.
Lichtman and colleagues from Harvard and Duke universities gave results of their study Wednesday at the American Heart Association's International Stroke Conference in San Diego. It is the largest and most detailed research on this issue.
Each year, about 800,000 Americans have a stroke. Most are due to clots that block a blood vessel to the brain, and high blood pressure is a major risk factor.
Some earlier studies found a seasonal trend to stroke rates, and there are biological reasons to think they are related, said one independent expert, Dr. Andrew Stemer, a neurologist at Georgetown University.
Blood vessels constrict in cold weather, which can raise blood pressure, he said. Extreme weather can trigger a stress reaction by the body, causing it to release substances "that not only increase the work of the heart" but make blood stickier and more likely to clot, Stemer said.
In cold weather "your body clamps down, there's cardiovascular stress," said Dr. Larry Goldstein, a Duke stroke specialist who worked on the study.
Conversely, "high humidity may cause dehydration," which also can raise the risk for clots and raise stress on the body, he said. "You know how you feel when you're out in hot, humid weather - you don't feel so hot."
Several of these same researchers published another study earlier this year that looked at stroke deaths from 1999 to 2006 among Medicare patients and found a pattern - higher rates in the winter, lower in summer and a small peak in July.
The new study looked at stroke hospitalizations, not just deaths, in a wider population of adults using a federal database covering all states except Idaho, North Dakota, Delaware and New Hampshire. Researchers also had daily climate data down to the county level from the National Climatic Data Center for 2010 and 2011.
Researchers tracked only strokes caused by clots, not the less common kind caused by a burst or bleeding blood vessel.
Lower temperatures, larger daily temperature changes and higher dew points (humidity) were tied to higher stroke hospitalization rates.
Each 5-degree increase in daily temperature fluctuation (the highest reading minus the lowest one) raised the chance of stroke hospitalization by 6 percent. Each 5-degree rise in the dew point (humidity) raised the risk by 2 percent.
The researchers did not establish a threshold when things were too hot - the point of the study was tracking the general trend, Lichtman said.
The results mean that during extreme weather, friends and relatives should "keep an eye on people that are at high risk, those who are older," she said.
During stressful weather conditions, "you want to watch your diet, watch your salt intake, regardless of what the temperatures are," and get enough fluids, said Daniel Lackland, a scientist at Medical University of South Carolina in Charleston.
Goldstein added this advice for people already at cardiovascular risk: "Stay in air conditioning in the summer and stay heated in the winter," so the weather outside affects you less.