Splash Park was a popular place Sunday as children and adults played in the cool water. Tucson is about to exceed 100 degrees for the first time in 2008.

Photos by Jeffry Scott / arizona daily star

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The forecast says it will hit 100 in the next couple of days in Tucson, eight days ahead of the 112-year average of May 26 if it happens today — but pretty much right on time for this decade.

Last year, we hit 100 on May 11, and the average occurrence of the first 100-degree day this decade is May 17.

It's getting warmer in Tucson, by day and by night, and the weatherman says don't bet on a reversal of that trend.

In a recent report for the National Weather Service, meteorologist John Glueck concludes that it's getting harder and harder to set new low-temperature records.

Achieving them as a monthly or yearly average is "nearly impossible."

In one dramatic example, Glueck used historical data to calculate the odds of hitting Tucson's all-time record low for July (49 degrees on July 3, 1911) at 1 in more than 6 billion — 6,215,204,907, to be mathematically precise.

That 1911 reading was a bit of an anomaly — the 49 degrees recorded was well below the average July low temperature of 73.

Overall, though, it's clear that our record lows, most of them set in a much earlier era, have become, if not impossible, at least very difficult to break.

Glueck attributes most of the increase in overnight lows to the urban-heat-island effect. Our built environment absorbs and retains more heat than the natural one and radiates it through the night. As we grow larger, our climate grows warmer.

Studies compiled by the Environmental Protection Agency show the heat-island effect can make urbanized areas two to 10 degrees warmer than adjacent rural areas.

Climate change also may be at play, Glueck says, but his study focused on the heat island and shows a direct correlation between the growth of Tucson and the rising overnight lows, a trend he traces to the mid-1960s.

Only nine record-low daily temperatures have been recorded since 1980.

If you want to bet on a record, Glueck said, put your money on the upper end of overnight temperatures, or "high lows" as they are called. Those are rising.

We have set nine such marks since 2000, he said, including July 22, 2006 when the temperature never dropped below 89. Last year, on July 5 and 6, it never fell below 87.

Sometime soon, Glueck said, Tucson will record a day that never drops below 90.

If you live in Phoenix, Glueck said, rest assured that someday soon the temperature will stay above 100 for an entire day and night. The current Phoenix record is a low of 96 on July 15, 2003.

If the trend continues, we may have trouble answering that question people ask: "How do you stand the summers there in Tucson?"

"The nights are wonderful" works, to a point, but it's being threatened by this trend of higher lows.

"It's a dry heat" was never very convincing, and it's an outright lie for half the summer.

Guess we could just tell people: "We love being above average."