RELEASE: 01-141 - July 12, 2001

GREATER SOLAR ACTIVITY MAY BRING U.S. MORE GRAY DAYS

David E. Steitz Headquarters, Washington, DC (Phone: 202/358-1730) Cynthia M. O'Carroll Goddard Space Flight Center, Greenbelt, MD (Phone: 301/614-5563)

NASA-funded Earth Science researchers have discovered that during periods of increased solar activity much of the United States becomes cloudier, possibly because the jet stream in the troposphere moves northward causing changes to regional climate patterns.

The new study supports earlier findings by suggesting there is a relationship between increased cloud cover over the United States and the solar maximum, the most intense stage of activity on the Sun.

Previous studies have shown that during the solar maximum, the jet stream in the Northern Hemisphere moves northward. The jet stream guides storms and plays an important role in cloudiness, precipitation and storm formation in the United States.

Dr. Petra Udelhofen, a NASA-funded researcher at the Institute for Terrestrial and Planetary Atmospheres at the State University of New York at Stony Brook, is the lead author of a paper that discusses this topic, appearing in the July 1 issue of Geophysical Research Letters.

"Based on these results and because the location of the jet stream influences cloudiness," said Udelhofen, "we suggest that the jet stream plays an important role in linking solar variability and cloud cover."

The jet stream is a ribbon of fast-moving air in the upper troposphere that blows from west to east. Storms beneath the jet stream follow its path. A shift in the jet stream can alter the location of clouds and precipitation across the U.S.

The troposphere is the region of the atmosphere that extends from the Earth's surface out to about 50,000 feet and is the focus of local, regional and global weather research. The stratosphere extends above the troposphere to about 150,000 feet and is the region where the ozone layer is formed.

The Sun's energy output varies over an 11-year cycle, sending more ultraviolet radiation towards the Earth during times of increased activity. While the Sun's total energy output only varies by about one-tenth of one percent between periods of low and high solar activity, the ultraviolet radiation that affects ozone production in the stratosphere can change by more than 10 percent.

Ultraviolet radiation is absorbed in the Earth's stratosphere and creates the protective ozone layer. When the ozone absorbs ultraviolet radiation, it warms the stratosphere, which may affect movement of air in the troposphere where clouds form.

"Our results show that cloudiness varies on average by about two percent between years of solar maximum and minimum. In most parts of the U.S., cloud cover is slightly greater in years of solar maximum," noted Udelhofen.

Though more investigation is needed to better understand just how changes in the Sun's ultraviolet energy output is linked to atmospheric winds, the study helps people identify potential large-scale mechanisms that affect local and regional climates.

Scientists continue to investigate mechanisms that may link solar variability with weather. These new results support the idea of a link between stratospheric chemistry and meteorology, and support other recent theoretical studies associated with the impact of stratospheric chemistry on climate change and weather.

"It is important for future studies to identify and explain in detail the link between solar variability, ozone, the atmospheric circulation and cloud cover," Udelhofen said.

This research is part of the NASA Earth Science Enterprise program, which is dedicated to understanding how Earth is changing and what consequences these changes have for life on Earth.

More information is available on the Internet at: http://www.gsfc.nasa.gov/topstory/20010712cloudcover.html

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