After a day in which the humidity plunged to desert levels, a plant-threatening freeze was possible early Friday in much of the Philadelphia region, the National Weather Service warned, and yet another fire watch is in effect until late Friday night.
The freeze warning included northern Delaware and covered all but Philadelphia and Delaware County, which are more urbanized and tend to be slightly warmer at night.
The wild card in the other counties would be the amounts of cloud cover and wind, said Jonathan O’Brien, a meteorologist at the National Weather Service Office in Mount Holly.
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Clouds and winds inhibit daytime heat from escaping into space. They were factors Thursday morning, he said, when the entire region remained above freezing. Officially, at Philadelphia International Airport, where the open spaces typically are windier, it didn’t get below 40.
The winds were due to drop off Thursday night, and that could allow readings to drop below 32 away from the city.
“If the winds were going completely calm,” O’Brien said, “we would be looking at lows in the 20s.”
Meanwhile, the air continues to be exceptionally dry.
The humidity in Philly dropped to 16% late Thursday afternoon — that’s desert territory, and extraordinary given the proximity of so many bodies of water. Based on an analysis of records dating to 1948, the humidity has dropped into the teens in Philadelphia during about 0.2% of the hours.
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The weather service had issued a “red flag” warning for brushfire danger Thursday, but no significant fires were reported. It has a fire “watch” in effect until 10 p.m. Friday, when a warming trend should begin.
Temperatures are forecast to climb into the 60s Friday, Saturday, and Sunday.
And Thursday’s chill did some good, according to National Allergy Bureau pollen counter Donald Dvorin, with the Allergy and Asthma Doctors, in Mount Laurel. The counts dropped 60% from Wednesday’s “extreme” levels to just plain “high.”
The dryness also should be an asset for early-rising sky-watchers come Saturday morning. Bright Jupiter and Venus will appear almost conjoined in the southeast sky around 5 a.m., and with clear skies and no moon or water-vapor interference, they should be quite evident. EarthSky.org advises they also will appear only slightly farther apart on Sunday morning.