Records may fall

Regardless of just how hot it gets this afternoon, it's likely that Philadelphia will set a temperature record of some kind during this hot spell.

First, about the nights.

The National Weather Service is forecasting an overnight low of 78 for tomorrow morning. The all-time high minimum for a June 22 is 80 at Philadelphia International Airport, so that would be a close call.

The forecast low for Friday morning is 77, and the record high minimum for the day is 75. Any late-day shower, however, could bring down the temperature.

We mention the nights first because health experts fret more over hot nights than hot days. Those nights are especially dangerous in those rowhouse neighborhoods where elderly people live alone without air-conditioning.

A hot night makes the job of daytime heating all the easier, and those houses can warm up in a hurry once the sun comes up.

Generally, the nights have been getting warmer, and that could be a function of background worldwide warming.

Warmer air can hold more moisture, and that water vapor inhibits cooling by preventing heat from radiating into space after dark.

From 2002 to 2011, the average nighttime low in Philadelphia in summer was a toasty 67.5. By comparison, the average low in the 1960s was 64.9.

As for the days, today's forecast high is 97, and that will make a run at the record, 98. Tomorrow, the forecast high of 99 would match the record for the date.