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SALT LAKE CITY (AP) -- Summer nights in Salt Lake City have become hotter in the past 50 years, a newspaper's study concluded.
The lows have gotten progressively higher each decade, even as daytime temperatures have lowered a tiny bit, the Deseret Morning News said in a copyright story Friday.
The newspaper looked at data between 1956 and 2006, and found that during the 25-year period between 1981 and 2005, the minimum summer temperature in Salt Lake City was 3.2 degrees higher than during the previous 25 years. August nighttime temperatures jumped 4.1 degrees over that time.
"Three degrees over 10,000 years would be expected, maybe," said Jim Ashby, a climatologist with the Western Regional Climate Center in Reno, Nev. "But 3 degrees in 25 years is very significant."
In St. George, the average nighttime July minimum was 72 degrees in the 1981-2005 period, compared to 68 during the 25 years previously. In the past 10 years, the average nighttime low in July was 74.
Rising urban nighttime temperatures have been observed worldwide, and various explanation have been offered.
The warmer nights in Salt Lake City could be an "urban heat island effect," said meteorologist Brandon Smith at the Salt Lake of the National Weather Service. "You put in more concrete and asphalt, and it holds in the heat more than vegetation, sand and salt does."
But Kelly Redmond, deputy director of the Western Regional Climate Center, said the temperatures are recorded at Salt Lake City International Airport, which is away from the city and upwind, "so this should not be a major factor."
Another theory is the heat is held in by increased cloudiness due to jet contrails.
"We know that the Earth is much cloudier today than it was 50 years ago," said David J. Travis, professor of geography and geology at the University of Wisconsin-Whitewater. "And no doubt some of this is an increase in high clouds that can partially be attributed to jet contrail increases."
A study by Travis and two colleagues found that when commercial airlines were grounded for three days after the Sept. 11, 2001, terrorist attacks, the days became warmer and the nights cooler. The range between the two was about 2 degrees more
The results suggested that contrails keep daytime highs lower, by reflecting sunlight back into space, and also raise the nighttime lows, by keeping radiated heat in.
Redmond said greenhouse gases like carbon dioxide and methane "are also expected to help increase nighttime minimums."
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Information from: Deseret Morning News, http://www.deseretnews.com
(Copyright 2006 by The Associated Press. All Rights Reserved.)
