July 20, 2013

There’s a way of coping with day after day of temperatures in the 90s, besides staying in the shade, wearing loose clothing and drinking lots of water. It is to go home, let the sun set and feel the swelter subside.

But that traditional method of riding out a heat wave has not been particularly effective in recent days for those living close to the Washington area’s urban core near Reagan National Airport, where the National Weather Service takes the city’s temperature.

It has been hot during the day, yes. Saturday, with a high of 94, was the sixth consecutive day on which Washington’s official temperature was well above 90. But even beyond each day’s heat and humidity, Washington’s weather woes were made all the worse by how warm it remained at night.

In fact, Friday set a record for staying warm. With a kind of stubborn thermal tenacity, when the sun went down the night before, the mercury stayed up.

The pre-dawn hours, which are generally the coolest of the day, remained warm. The low temperature Friday morning was 81 degrees. The temperature eventually rose to 95. But it never dipped below 81.

Never before in all the years that official weather records have been kept for Washington had there been a July 19 on which the temperature remained so high.

Such temperatures are not mere numerical quirks, meteorological museum pieces, destined for storage in a book of statistics.

It’s common knowledge that high humidity combines with high temperature to produce maximum discomfort.

But that does not make it necessary to ignore the less publicized contribution that elevated overnight temperatures can make to midsummer unpleasantness.

Particularly for people lacking adequate access to air conditioning, cooler overnight temperatures can make it easier to sleep, and so help to replenish the energies needed to withstand long spells of searing daytime heat.

Last week limited the possibility of any overnight restoration of energies. Friday set a record for staying warm, and Wednesday and Thursday matched existing records.

Neither day ever got cooler than 80.

However, by Saturday the series of record-setting or record-matching days was over.

On Saturday morning, the low temperature was 80 degrees, set at 4:18 a.m. It had been a warm night, but it had not set a record.

Saturday morning’s 80-degree low temperature was one full degree below the stay-warm record for July 20, which was set in 1980, a day on which the mercury never descended below 81.

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