Why it’s been so foggy in South Florida this week, alert issued this morning

The National Weather Service issued a special weather statement this morning warning of widespread patchy fog throughout South Florida, including western and coastal Palm Beach County.

Some isolated areas are experiencing dense fog where visibility is down to around a half mile, or even less.

“Motorists should exercise caution this morning across all of South Florida,” the statement says. “A gradual clearing through is expected through mid morning.”

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Fog can be caused when the temperature lowers to the dew point temperature and when winds are low.

Observations taken at Palm Beach International Airport this morning, show the temperature plunging in the early morning hours to nearly equal the dew point temperature of about 63 degrees. Dew point measures the amount of water vapor in the air. When the dew point temperature and air temperature meet, there is 100 percent saturation.

At the same time winds this morning are blowing very lightly out of the northwest, allowing the moisture to settle.

The combination contributed to the fog, which is a surface-level stratus cloud. (Note in the second box the cloud level significantly drops during when the fog is present.)

Fog is simply a low cloud in contact with the surface.

The temperature (purple line) and dew point temperature (green line) nearly met early this morning, helping create fog.

The temperature (purple line) and dew point temperature (green line) nearly met early this morning, helping create fog.

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Dense fog is rare in Florida. Between 2000 and 2011, NOAA’s storm events database recorded just 35 days during which dense fog was present statewide. But those incidences can be lethal.

The database lists five deaths and 17 injuries directly attributable to dense fog, while Palm Beach Post archives list multiple car accidents in which fog was involved.

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