- The Washington Times - Monday, September 15, 2003


Climate researchers studying records at thousands of locations have discovered that, in many communities, the temperature range between the daily high and low changes on the weekend.

In regions such as the Southwest, the Carolinas and Georgia, Sunday and Monday had a consistently larger daily temperature range than other days, with Fridays being the day with the smallest difference between high and low.

In many communities the difference in range between weekend and weekdays was nearly 1 degree Fahrenheit.

“The beauty of this weekend effect is it necessarily has to be of human origin, because we don’t have something in nature that cares whether it’s Tuesday or Saturday,” said Piers M. de F. Forster of the National Oceanic and Atmospheric Administration Aeronomy Laboratory in Boulder, Colo.

The researchers speculate the cause may relate to materials released into the air that help form clouds. Moisture in the air condenses more readily when it has something to adhere to, such as a tiny bit of dust or a chemical particle..

Clear nights are generally cooler than cloudy ones because clouds tend to reduce the amount of radiation lost by the land into space, and observations have shown that adding particles and aerosols to the air — over oceans for example — can increase cloudiness.

Mr. Forster, who is also affiliated with the University of Reading in the United Kingdom, and Susan Solomon of the NOAAL noticed the weekend effect while studying records in an effort to learn more about global warming.

Their findings were published in this week’s online issue of Proceedings of the National Academy of Sciences.

Some 35 percent of locations experienced a significant weekend effect over 50 years of record keeping, the researchers found.

But the weekend effect wasn’t always the same.

Many localities in the Midwest had reverse effects, with smaller temperature ranges on the weekend than weekdays. In those regions Tuesdays and Wednesdays typically had the biggest difference between the daily high and low.

“We were surprised, particularly in the Midwest … because we thought we would find Saturday and Sunday to have a bigger range than weekdays like the East Coast and West Coast, but it was the opposite,” Mr. Forster said. “We don’t know what’s going on.”

Mr. Forster and Miss Solomon found the weekend effect in New Mexico, Arizona, the Midwest and some Eastern states tended to be larger in the summer than winter.

So why does the weekend effect reduce the weekend temperature range in some places and increase it in others?

The researchers aren’t sure, though they have theories.

While pollution and dusty aerosols in some areas provide nuclei for water to condense on to form clouds, in other places there may be soot in the aerosols, which could absorb heat and cause the cloud to burn off — evaporate — during the day leaving less to warm the night, Forster suggested.

Another possibility might be that pollutants that warm the air could cause changes in wind circulation patterns on a weekly basis.

Or, the scientists said, there may be a gradual change across the country because of the downwind transport of pollutants from place to place. And they can’t rule out the possibility there is some other human-related mechanism at work other than pollution, aerosols and clouds.

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