As the jolt in those electric bills will affirm, officially this was the warmest June in Philadelphia in 137 years of recordkeeping.

And while the rest of the workweek should be comfortable, a potentially powerful heat wave may get under way as early as Sunday, with temperatures perhaps approaching triple digits.

But first, the heat that was.

The average June temperature, 78.2, beat the old record of 78.0, set in 1925 and 1994.

The temperature hit 90 or better 15 times, tying a record, but oddly, not a single high-temperature record was broken in June. The problem was, it just wasn't cool.

The nights were steamy, probably due to high humidities, and that could make a significant difference when the utility bills come around. The planet cools by radiating heat into space after the sun goes down, but water vapor gets in the way.

The average low temperature for June, 68.8, tied 2005 for the second-highest monthly minimum on record. The 69.2 of 2008 was the warmest ever.

While various studies have shown that nighttime warming has accelerated as the planet has become warmer, Stanley A. Changnon, a University of Illinois climate expert, cautioned against reading too much into the thermometer at Philadelphia International Airport.

He and Tony Gigi, a meteorologist at the National Weather Service office in Mount Holly, said the thermometer could be responding to urbanization effects, given the development all around it.

Still, it has been unquestionably warm, and Gigi noted that June was a particularly humid month. That was not a good development for air-conditioning users.

"The hotter and more humid, the more inefficient the cooling device will work," said Peco Energy Co. spokesman Ben Armstrong.

On average, Peco's peak loads were better than 20 percent higher in June, compared with last year, according to Peco figures.

After this week's respite, expect the region's air conditioners to reach peak hum next week.

The European computer forecast model shows temperatures approaching 100 Sunday and perhaps hitting 105 midweek, Gigi said.

The U.S. forecast calls for the relatively comfortable mid-90s.

Contact staff writer Anthony R. Wood at 610-313-8210 or