Heat's dark side: When the days ARE like the nights

To paraphrase songwriter John Sebastian, this season it's a pity that the days have been like the nights in the summer, in the city.

The overnight today didn't get below 79, and through the first 71 days of the meteorological summer that began June 1, the daily "lows" are an all-time high, averaging 70.9.

Barring a change that so far hasn't shown up in the extended outlooks, the daily minimums are going to torch another record.

Never have the minimums averaged 70 or above for the June 1-Aug. 31 period. The standing record is 69.3, set in 1994, followed tightly by 1995, 69.2; 2005, 69.1; 1993, 68.8, and 1931, 68.5.

On average, starting today the temperatures back off subtly as the nights get longer. The normal low is 70 on Aug. 10 and backs down to 66 by the end of the month.

But avoiding a record this year isn't going to be easy. Looking at the other years on the top five list, in three cases the average minimums for the three-month periods actually increased after Aug. 10. In 1995 the average stayed the same.

It dropped 0.5 degrees in 1994, but even if that were matched this year, the summer still would set a record of minimum temperature.

Why have the nights been so warm in recent years? Urbanization is a factor. Paved surfaces and buildings soak up heat in daytime and are reluctant to give it up after dark. 

Another factor could be a general increase in water vapor as a result of worldwide warming. That vapor inhibits cooling.

The warm nights mean hotter mornings and jump-starts starts to daytime heating. For most of us they mean more discomfort; for the elderly and vulnerable, perhaps danger.