Adolescents' physical activity drops during winter and increases during summer — but not enough to offset the winter decline, study findings indicate.

The net result is that "as adolescents get older, their physical activity levels drop drastically," Dr. Mathieu Belanger, of the Universite de Moncton in New Brunswick, Canada, told Reuters Health. In fact, physical activity declined "one-third in our five-year study," Belanger said.

As described in the Annals of Epidemiology, Belanger and colleagues followed seasonal physical activity variations of 1293 Montreal-area students who were 12 to 13 years old at the start of the study. The investigators assessed students' physical activity levels every 3 months over the 5-year study and compared these data with local weather conditions.

The researchers found an association between weather conditions and the students' physical activity levels.

For example, the average number of physical activity sessions reported daily was 2 to 4 percent lower for each 10 millimeters (0.4 inches) of recorded rainfall. Every 10 degree Centigrade rise in temperature (18 degrees Fahrenheit) was associated with 1 to 2 percent greater physical activity.

Belanger's group also noted 5 percent higher activity rates linked with each 10 centimeters (4 inches) of snow accumulation, but a drop in activity on days when snow fell.

"Physical activity was lower during winter and increased during warmer months. However, the warm-month increases did not compensate for winter decreases so that activity decreased by 7% yearly," the researchers report.

"With prolonged periods of low physical activity levels, youth have a hard time getting back to being sufficiently physically active," Belanger said.

"These results should be sufficient to motivate public health authorities to invest in efforts to get youth more physically active during winter months," he added.