WASHINGTON (AP) _ Life is different on weekends, a distinction that seems to affect Mother Nature as well as people.
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.
And, as with some people, there seems to be a little hangover of this weekend effect on Mondays.
Piers M. deF. Forster and Susan Solomon of the National Oceanic and Atmospheric Administration Aeronomy Laboratory in Boulder, Colo., noticed the weekend effect while studying records in an effort to learn more about global warming.
Their findings are 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.
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.
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,'' said Forster. ``We don't know what's going on.''
Forster and Solomon found the weekend effect in New Mexico, Arizona, the Midwest and some Eastern states tended to be larger in the summer than winter.
The researchers concluded the effect must be a result of human activity, an important link for scientists who already had evidence that overall temperatures had increased over the past century or so.
``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 Forster, who is also affiliated with the University of Reading in the United Kingdom.
Since the change in daily temperature range was mostly a result of nighttime minimums on some days not being as low as on other days, 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 since clouds tend to reduce the amount of radiation lost by the land into space, and observations have shown than adding particles and aerosols into the air _ over oceans for example _ can increase cloudiness.
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 added, they can't rule out the possibility there is some other human-related mechanism at work other that pollution aerosols and clouds.