Behind the Forecast: When freezing fog leads to snow

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Science Behind the Forecast 1/18/19

LOUISVILLE, KY (WAVE) - With temperatures commonly near freezing at this time of the year, freezing fog is an issue, especially in regards to travel.

Freezing fog forms exactly like normal fog does. When skies are clear, heat from the Earth’s surface is radiated into space allowing temperatures to drop. As temperatures drop to meet the dewpoint, moisture in the air is forced to condense onto whatever condensation nuclei is in the atmosphere, leading to fog. Fog can also form when warm air moves over a colder area of land or water, air is pushed up a mountain and cooled or when warm air mixes with a colder air mass air above.

When temperatures are below freezing, the minuscule water droplets in the fog become super-cooled; meaning the stay liquid even those temperatures are below 32°F. Once they get into contact with any surface, they freeze. This typically causes rime ice which is white because of the air trapped inside because of the rapid freezing process.

The freezing fog leading to a "snowfall" look this morning. The rime ice is coating everything. Even the roads, so drive safely! (pic from Molly Lundy in Okolona)

Posted by WAVE 3 Weather on Wednesday, December 21, 2016

If the fog has ice crystals instead of water droplets then its called ice fog.

This causes problems when you factor in the roadways. If pavement temperatures are below freezing then black ice will easily form.

Aircraft must be treated or have good de-icing equipment to prevent ice accumulating on the plane and causing a dangerous situation.

Dense freezing fog can actually lead to snowfall. As a fog layer forms into a stratus cloud and temperatures drop, not only do super-cooled water droplets form but also ice crystals. The ice crystals grow at the expense of the water droplets on condensation nuclei leading to small snowflakes. The snowflakes continue to grow until they fall.

Freezing fog in our southern counties this morning created glistening ice on trees. Check out these photos from our viewers! -Ryan Hoke

Posted by WAVE 3 Weather on Sunday, January 24, 2016

This actually happened in Louisville back on December 21, 2016. Louisville picked up 0.1″ of snow during the freezing fog which was the first measurable snowfall of the season.

A Freezing Fog Advisory will be issued by the local National Weather Service office in Louisville when dense fog develops and surface temperatures are at or below freezing with visibility around a quarter of a mile.

The video below, sent in by a WAVE 3 News viewer Danny Hernandez to Meteorologist Brian Goode, shows the snowflakes falling from the fog back in December 2016 off Strawberry Lane.

From Danny Hernandez off Strawberry Lane. Flakes flying from the freezing fog this morning. Some saw rime ice as well. Very wicked to see. Welcome to winter :)

Posted by Brian Goode WAVE 3 News on Wednesday, December 21, 2016

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