Hot time — summer in the city

You can look at the statistics or you can sleep in a bedroom without air-conditioning. Either way, you might stumble upon a meteorological phenomenon: Summer nights in Salt Lake City have become hotter in the past 50 years.

The lows have gotten progressively higher each decade, even as the daytime temperatures have cooled a tiny bit. During the past 10 years, the average low on a summer night was 62.3 degrees — which doesn't sound so bad until you remember that this is the number finally reached just before sunrise, after you've already tossed and turned for hours while the temperature lingered in the 80s.

The Deseret Morning News 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 hotter 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," says Jim Ashby, a climatologist with the Western Regional Climate Center in Reno, Nev. "But 3 degrees in 25 years is very significant."

If you compare the decade 1956-1966 with the decade 1996-2005, the difference is even more striking. It's now 4.5 degrees hotter at night than it was then, 5 degrees if you compare just the August numbers.

Kelly Redmond, deputy director of the Western Regional Climate Center, happened to be in Salt Lake City during the first week in June and observed the hot nights firsthand. "It never cooled down," Redmond remembers about the night of June 7- 8. Later he looked at the numbers and discovered the low that night was 69, "which is very warm," he notes, "and actually is the warmest June 8 minimum in the last 58 years."

In St. George, summer nights now are 3.9 degrees hotter than before. That means an average nighttime July minimum of about 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.

In the six years between 2000 and 2005, notes Redmond, Salt Lake City set 36 "daily minimum" records (in other words, high lows). That's four times as many as might be expected if they were randomly distributed throughout the 58 years data has been collected, he says.

The reason for the hotter nights — a phenomenon that is also happening in other cities around the world — isn't entirely clear. Meteorologists offer explanations but don't necessarily agree among themselves. The upshot, though, seems to be this: Too much man-made stuff on the ground, too much man-made gunk in the air.