Dr. Annick Alperovitch of the Institut National de la Sante et de la Recherche Medicale in Paris and colleagues assessed the relationship between blood pressure and temperature in 8,801 individuals age 65 or older.

All were part of the Three-City study, conducted in three French metropolitan areas.

Both systolic, or the top number, and diastolic, or the bottom number, blood pressures differed across the four seasons and across the distributions of outdoor temperatures.

The study, published in the Archives of Internal Medicine, found the average systolic blood pressure was 5 millimeters of mercury higher in winter than in summer.

High blood pressure -- defined as a systolic blood pressure of 160 millimeters of mercury or higher or a diastolic blood pressure of 95 millimeters of mercury or higher -- was detected in 33.4 percent of participants during winter and 23.8 percent during summer.

"Because the risk of stroke or aneurysmal rupture is highest in the elderly, improved protection against these diseases by close monitoring of blood pressure and antihypertensive medication when outdoor temperature is very low could be considered," the authors said in a statement.