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Sizzling sun makes cloudy days

When the sun flexes its ultraviolet muscles, the United States goes into a fog
When the sun flexes its ultraviolet muscles, the United States goes into a fog  

By Richard Stenger

(CNN) -- The United States becomes cloudier during cyclical periods of heightened solar activity, possibly because the increased radiation heats the upper atmosphere and nudges the jet stream northward enough to change regional weather, researchers said this week.

Their study, which investigates a link between weather and the composition of the stratosphere, could help scientists identify large-scale mechanisms that influence climate.

The jet stream, a belt of powerful wind that gusts from west to east in the troposphere, the lowest atmospheric layer, exerts a major influence in directing storm fronts across North America. Jet stream fluctuations can determine which locations soak up sun and which soak up rain.

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

The work supports previous observations finding a connection between more clouds and the peak of an 11-year cycle during which the sun unleashes more potent bursts of energy. Directed toward Earth, such eruptions can disrupt satellites and power grids.


The new study suggests that during the so-called solar maximum, as the sun cranks out more ultraviolet radiation, the upper atmosphere boosts production of the protective ozone layer, which in turn absorbs more ultraviolet waves, heating up the air at high altitudes.

The Sun's overall energy output differs by about one-tenth of one percent between periods of minimum and maximum solar activity. But the ultraviolet radiation that affects ozone production in the stratosphere can vary by more than 10 percent, the scientists said.

The stratospheric warming could influence air patterns in the troposphere just below it, where the jet stream resides and clouds form, the NASA-funded researchers said.

"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," Udelhofen said.

Udelhofen and colleagues reported their findings in the July 1 edition of the journal Geophysical Research Letters.

• Interactive Planetary Orbits
• Solar Dynamics Observatory
• The Johns Hopkins University
• Living With a Star Program

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