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U.S. weather waxing cloudy.

U.S. weather waxing cloudy

Do you remember the weather years ago as having beensunnier than now? You may not be idealizing the past; you may be right. Two current studies, one an analysis of overall cloudiness in the United States and the other a close look at the weather of Michigan, indicate that the United States has gotten cloudier. Previous work had shown that the central United States has suffered the same fate.

William L. Seaver of Virginia Polytechnic Institute in FallsChurch and James E. Lee of the MITRE Corp., a nonprofit systems engineering company in McLean, Va., compared the number of cloudless days in 45 U.S. cities during two periods, 1900-1936 and 1950-1982. Cloudless days were defined as those in which an average of 10 percent or less of the daytime sky was obscured clouds, haze, smoke or fog. The study used data collected by U.S. National Weather Service observers.

An increase in cloudiness in the middle third of the UnitedStates has been documented by Stanley Changnon and his colleagues at the Illinois State Water Survey at the University of Illinois in Urbana-Champaign, but according to several meteorologists, Seaver and Lee are the first to show the trend for the nation as a whole.

The two researchers report in the current (January) JOURNALOF CLIMATE AND APPLIED METEOROLOGY that the second half of the century has had fewer cloudless days than the first half. Los Angeles, for example, averaged 10 cloudless days per month in the years from 1900 to 1936; from 1950 to 1982, the number of cloudless days dropped to 7.6 per month. St. Louis went from 7.2 to 4.7 cloudless days per month, and Washington, D.C., from 5.3 to 4.4. Of the 45 cities checked, the only one to get sunnier was Ft. Worth, Texas, but the increase from 7.4 to 7.5 was barely enough to be significant, Lee says.

And at the Fourth Conference on Climate Variations inBaltimore last week, Val L. Eichenlaub of Western Michigan University in Kalamazoo presented data showing an increase in cloudiness in Michigan. Grand Rapids, for example, was 75 to 80 percent sunny in the late 1930s and early 1940s but dropped to about 65 percent in the 1970s.

While the studies have shown a trend toward cloudiness,they don't explain why the change is occurring. But the researchers involved in the work have several ideas. Changnon, who collected the Midwest data, has suggested that jet contrails act as condensation "seeds' and instigate cloud formation. For Michigan, Eichenlaub says, other data indicate that the polar weather front has been shifting southward, and this could be pulling in more storms and clouds. On the national scale, Lee suggests that pollution could be supplying particles around which water may condense.
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Publication:Science News
Date:Mar 28, 1987
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