The fog reduced visibility to nearly zero in some locations.
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The Sky6 cameras were proof of that.
The fog made the Philadelphia skyline disappear at times.
According to AccuWeather: "Fog is essentially a low cloud and can either form in place or physically move into an area."
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The National Weather Service issued a dense fog advisory for the entire region until 10 a.m.
"Not every locale is seeing dense fog but throughout the region there are areas of dense fog with visibility near or below a quarter mile," the NWS said.
The fog will give way to some sunshine for the rest of Friday. Then the rain moves in for Saturday.
AccuWeather: Some sun, still warm