July among warmest on record, and, oh, those nights

Last month was the eighth-warmest July in Philadelphia in official records dating to 1874.

And in a pattern evident in recent summers, something colleague Tom Avril and I wrote about during convention week, the nighttime warming outpaced that of the days.

Averaged out over the 31 days, the high was 89.9. Among all the Julys in the period of record, that was the 12th-highest for July's maximum temperature.

The minimum average temperature was 72.5, and that was the sixth-highest such reading on record.

Why has warming been more robust at night than in the daytime? Background worldwide warming is a prime suspect; theoretically, a warmer atmosphere can hold more water vapor.

Vapor inhibits warmth from escaping into space after sunset.

Another factor would be urban warming. All those buildings and paved surfaces absorb warmth during the day and are reluctant to give it up at night.

The official station at Philadelphia International Airport almost certainly has been affected by the urban heat island effect.

Whatever the cause, a lack of overnight cooling is an especially hazardous development in rowhouse neighborhoods where houses can become brick ovens after sunrise, endangering the lives of the old and vulnerable who don't have air-conditioning.

Here are the six Julys with the warmest minimum temperatures averaged for the month. Note the preponderance of recent years.

1994: 73.4