Only hours after unseasonable warmth that pushed temperatures into the 80s, a FROST ADVISORY is in effect for all the counties surrounding the city of Philadelphia Thursday night into early Friday.
Much colder air started moving in overnight Wednesday and 20 to 30 mph winds continued to feed in colder air during the day. Those northwest winds also brought in very dry air.
As the wind diminishes quickly Thursday night, conditions become more favorable for frost to form.
Frost requires clear skies, light winds and low humidity. This allows the cold air to sink to the ground and not be stirred up, while the heat from the day escapes into space. It is most likely in places away from cities, where there is less concrete. Concrete holds in heat, and that's why Philly is not in the advisory.
Sandy soils lose heat more efficiently than other types, so parts of South Jersey "radiate" very well under such conditions.
The big question overnight is whether the wind will become calm. It's pretty rare for that to happen after a day with winds of 15 to 25 mph and gusts to 30 mph. So, we'll be watching the wind speeds all night, along with the actual temperatures, to see if frost does indeed develop.
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