Steamy nights ahead

Those potent thunderstorms have pulled away, and before this heat surge is over, we may be rooting for encores.

We've mentioned the high dew points expected during the weekend, and how they are going to push head indices into the July-like 90s.

But the nights also are going to be heat-wave uncomfortable with overnight lows having a hard time breaking 70.

That is symptomatic of the aforementioned dew points, which will betray that the air is stuffed with moisture.

When the air is full of water vapor, it acts as a heat trap. Heat that builds up during the day has a hard time escaping into space after dark, and the temperature is slow to fall.

That's a bigger deal in the cities than in the outlying areas, because urban buildings and paving absorb solar heat that they are slow to release at night, making cities that much warmer.

The forecast low for Tuesday morning in Philadelphia is 72. That would match the record for the date for the highest minimum temperature.

Things should cool down after that. The Climate Prediction Center's 6-to-10-day outlook has temperatures averaging below normal.

We boldly predict that air-conditiong use will be above normal this weekend.

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