Without so much as a single daily high-temperature record, and blessedly absent of a deadly heat wave, the June 1-Aug. 31 period officially constituted the third-warmest meteorological summer in Philadelphia’s 150-year period of record.
The average temperature of 78.4 at Philadelphia International Airport finished behind only the 79.6 of 2010 and 78.6 of 1995, according to an Inquirer analysis.
Philadelphia recorded 36 days of 90-plus readings; the long-term average is 21. But it was what happened after sunset that put the quite eventful summer of 2020 near the top for heat, as has been the case in recent summers.
“A lot of that warming trend comes down to the overnight low temperatures,” said Jonathan O’Brien, a meteorologist with the National Weather Service office in Mount Holly.
The average minimum temperature for the three-month period, 69.9, was tied for second place. In New Jersey, statewide the nights were the warmest on record, said David A. Robinson, the Rutgers University professor who is the state climatologist. The data go back to 1895.
By contrast, Philadelphia’s average daytime high for the three months, 86.9, didn’t even make the top 10.
The only temperature record was for a high daily minimum, 79, on July 29. But three daily precipitation records fell, and that could be related to those warm nights.
The wetness paradox
While nowhere near a record, rain for the three-month period was significantly above normal: 17.28 inches compared with 11.28.
In an orderly universe, above-average rainfall would coincide with less sun and lower temperatures. However, the first four words of the previous sentence have no place in 2020.
The dry intervals allowed the sun to bake the ground sufficiently to produce that harvest of 90-plus days, said Carl Erickson, a meteorologist with AccuWeather Inc.
Hot nights in the city
Not that Philadelphia was Arizona East when the rain stopped.
The overall summer pattern was driven by persistent high pressure centered over the North Atlantic, the so-called Bermuda high, whose clockwise winds circulating around the center exported plentiful supplies of subtropical air.
Water vapor inhibits nighttime cooling by retarding the escape of daytime solar heating into space.
The trend toward higher dew points and humidity would correlate well with increases in the planet’s temperature. Warmer air can hold more moisture.
The net result in recent years has been a run of very warm nights but an absence of extreme, prolonged daytime heat. The longest heat wave this year lasted seven days, in July, and the temperature never got above 93 in August.
The Philadelphia Public Health Department said Wednesday that it had confirmed a total of four heat-related deaths for the summer, none of which occurred during city-declared heat emergencies. In the last decade, the city has recorded 87 heat-related fatalities; in the 10-year period that began in 1993, 399 deaths were blamed on heat.
“The range is lower” between daily highs and lows, said O’Brien. The average highs in the 21st century have been 2.2 degrees higher than those of the 20th century; the lows have been 2.7 degrees warmer.
In that sense, he added, Philadelphia’s summers have been “almost tropical.”