Overnight temperature Monday a record 81

The region has had hotter days than Monday - when it hit a steamy 95 at Philadelphia International Airport - but not many hotter nights.

In fact, the overnight low of 81 was the highest minimum temperature for a July 20 in Philadelphia since official record-keeping began 1874, beating the old record of 78 and missing the all-time mark of 83, set in 2010 and 2011.

Temperatures on Tuesday were forecast to go no lower than the mid-70s, but the heat wave should be rather unexceptional history by Wednesday.

Public health officials and meteorologists rightfully call attention to those triple-digit heat indexes, and the index did hit 101 at 2 p.m. and again at 2:30 Monday.

Records for high minimum temperatures tend to slip under the radar. But health experts have long warned that overnight warming is a bigger health threat than daytime highs.

Without nighttime cooling, the city's phalanxes of brick rowhouses warm up rapidly when the sun rises - creating a prime source of heat-related fatalities.

"If it doesn't get down into the 60s or 70s at night, what kind of chance do you have during the day?" said Chris Gallagher, director of the Heatline at the Philadelphia Corp. for Aging.

The line was activated for the first time this summer Sunday as the National Weather Service and the city declared their first excessive heat warning. Relative to recent summers, these last two have been benign. Through Monday, Philadelphia had recorded 11 days of at least 90 degrees this season, its long-term average through July 20. By comparison, it touched 90 a total of 25 times in 2012.

The first heat wave lasted just three days, ending on June 23. Technically, this one will persist Tuesday, however, humidity will drop during the day, and, said Kristin Klein at the weather service office in Mount Holly, "that will really feel nice."

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Inquirer staff writer Matt Gelb contributed to this article.