All wind and clouds on Earth are directly or indirectly tied to the Sun. Heat from the Sun produces the temperature differences that lead to pressure differences. Air naturally moves from high to low pressure areas, and this creates the winds.

The oceans store solar heat for long periods, and watery currents move the energy around the globe, fueling everything from mild breezes and localized fog to ferocious storms.

Consistent troughs of wind carry hurricane seedlings from Africa to Florida in a process that can take weeks. Days-long interactions between warm, moist tropical air and cooler Arctic air fuels tornadoes in the South and Midwest that sometimes migrate to the Northeast. A buildup of afternoon heat can force air to rise and fuel mountain thunderstorms that billow out of nowhere on a moment's notice. Earth's tilt and orbit conspire to alter how much sunlight reaches each hemisphere, creating the seasons.

Recent research shows that during the peak in the 11-year cycle of solar activity, at least some parts of the United States experience more cloudiness.

Udelhofen studied data from 1900 to 1987 collected by observers who noted the percentage of cloudiness several times each day. She also studied the records from dozens of automatic stations around the country that record the amount of sunshine each day.

Udelhofen then combined these data sets and compared them to the solar cycle, as measured by the number of sunspots.

Her work, published in the July 1, 2001, issue of Geophysical Research Letters, shows that cloudiness in much of the United States is about 2 percent greater during years of solar maximum compared to the solar minimum years.

Udelhofen suspects the increase is caused by a shift in the jet stream, a river of high-speed, high-altitude winds that circle the globe, west-to-east, in an undulating manner, sometimes staying well north and sometimes dipping deep into the United States.

Previous studies have indicated that the jet stream shifts north during periods of high solar activity. The shift is tied to increased energy in the atmosphere created by the absorption of the Sun's additional output.

And meteorologists know that storms track along the jet stream. Previous work by Joanna Haigh of Imperial College in London found that in the Mediterranean, storm tracks shift north by some 400 miles during solar maximum. It's uncertain though if there were more clouds created or if clouds were pulled north, stolen from the rest of North American and the Caribbean, as Udelhofen only looked at U.S. data.

Still, Udelhofen said she's provided the first observational data to connect the Sun's cycle to increased cloudiness that might be associated with this shift in the jet stream.

"Most of the continental United States shows an increase in cloudiness, but on the West Coast and also over the Great Lakes there is a decrease," Udelhofen said.

The study did not consider cloud type, and so no inferences can be made about possible changes in precipitation associated with the additional clouds.

In recent years, a growing number of scientists have suggested that changes in the Sun's output, and more precisely changes in its overall brightness, could be responsible for some or most of the global warming that has been measured.

The average surface temperature around the globe has risen by about 1 degree Fahrenheit since 1880. While much evidence suggests the rise is due largely to the output of carbon dioxide from cars and factories, many scientists have reserved judgement on whether, or at least how much humans have contributed to the temperature rise.

Might the Sun be the real culprit?

Since the 1970s, researchers have known that when there are more sunspots, the Sun is brighter. And mounting evidence shows a connection between this brightness and the overall warmth of Earth. The connection is related not just to the 11-year solar cycle, but to much longer periods of high and low solar activity.

Studying tree rings and ancient layers of glacial ice for clues to past global temperatures, researchers have found curious links to records of the solar cycle. Most interesting is what scientists call the Little Ice Age, a temperature drop that began in the 13th Century, bottomed out at 2 degrees below the long-term average, and did not reach previous levels until the late 19th Century.

Solar activity was persistently high prior to the 13th Century, when things were warmer, according to Sallie Baliunas, a researcher at Harvard-Smithsonian Center for Astrophysics.

"Activity then dropped to low levels during the Little Ice Age, and recovered by the early 20th century," Baliunas says. "The period of least solar activity coincided with the coldest century of the last millennium -- the 17th century."

Critics frequently charge that the Sun's total output does not change enough to affect Earth's climate so strongly. Baliunas says that's a good argument. "But that leaves unanswered the fact that the Sun's signal is so strong in the climate records."

Still, Baliunas doubts that the Sun is the sole cause of global climate change. She said current research seeks to figure out the magnitude of the Sun's influence so that the human effect can be better assessed.