The notion that the mercury plummets as the summer sun goes down is starting to seem as quaint as a Norman Rockwell painting.
The new-millennium United States is inundated with extra-hot summer nights, with government weather records showing triple the average occurrence of "much above normal" night minimum temperatures from 2001 to 2005.
Tucson weather data confirm the trend, with low temperatures last month and in July 2005 averaging 76.9 degrees, tied for the all-time-record warmest average monthly low. Last month also set a monthly record for the number of days with a low 80 or higher — 16 — and the warmest daily low, 89 degrees on July 22.
The U.S. Climate Extremes Index provides insight into the sweltering summer nights. "Much above normal" means low temperatures in the highest 10 percent on record, and during any given year, about 10 percent of the country should register "much above normal" summer-night lows.
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The portions of the country experiencing extreme summer night temperatures have skyrocketed recently. In 2005 and 2003, 36 percent of the nation had much-above-normal summer low temperatures. In 2002, it was 37 percent.
The single-year record came during the height of the Dust Bowl, when 41 percent of the nation had much-above-normal summer-night temperatures. However, the five-year average from 2001 through 2005 is a record by far.
And it's not just the last five years. Each of the past eight years has been far above the normal 10 percent. During the past decade, 23 percent of the nation has had hot summer nights. During the past 15 years, that average has been 20 percent. By comparison, from 1964 to 1968, only 2 percent of the country on average had abnormally hot nights.
A top federal research meteorologist said he "almost fell out of my chair" when he looked over U.S. night minimum temperature records over the past 96 years and was jolted by the trend toward hot summer nights.
"This is unbelievable," said Richard Heim, National Climatic Data Center research meteorologist. "Something strange has happened in the last 10 to 15 years on the minimums."
Increased suburban development and the "urban heat island" effect probably contribute to the extreme night heat, said Steven Mullen, chairman of the University of Arizona department of atmospheric sciences.
"Phoenix has incredibly warm temperatures during the evening, and it's because basically you have all that concrete around," Mullen said.
The number of extra-hot nights is not likely to be as significant in non-urban areas, Mullen said, but the data overall depend on where the observations are taken.
Scientists would have to examine global temperature patterns in the same way to know whether the trend exists worldwide, but "the bottom line is consistent with global-warming scenarios," Mullen said.
Joaquin Ruiz, dean of the UA's College of Science, said an increase in climate extremes is expected from global warming.
"That is what makes global climate change most worrisome," he said. "It's not that the whole Earth will be warmer by a few degrees; it's that the climate will get more extreme. Throughout the world we'll see areas where it rains more, where it's warmer, where it's colder."
On StarNet Watch an Associated Press slide show of people trying to cope with scorching heat, high humidity and a lack of air conditioning. azstarnet.com/multimedia
"This is unbelievable. Something strange has happened in the last 10 to 15 years on the minimums."
Richard Heim, National Climatic Data Center research meteorologist
