Why the fog was so persistent Tuesday morning

Fog at World Golf Village Tuesday from a SnapJAX viewer.

JACKSONVILLE, Fla. -- – Northeast Florida and southeast Georgia are well-known for fog in the winter months.

But this morning, the fog lasted much longer.

The Dense Fog Advisory that the National Weather Service issued lasted until 10 a.m., but areas of fog were constant through midday.

So, why so much fog for so long?

What causes fog

Fog is by definition a low cloud on the ground. But conditions have to be right to get fog for form.

The most important ingredient for fog formation is calm winds. Any wind can prevent fog from organizing and help “mix” the atmosphere.

We also need high moisture content in the air right at the surface. This measurement of moisture content in the atmosphere is called the dew point.

Moisture right at the surface is critical to fog development.

When the air temperature and the dew point are the same, the atmosphere has 100% humidity content and often fog begins to form.

Fog develops as the moisture turns into vapor and expands.

So, why did the fog last nearly all morning?

The inversion layer

The atmosphere is not the same throughout. In fact, the atmosphere has layers.

Almost all weather on Earth occurs in the lowest level of the atmosphere. This is called the troposphere.

The troposphere is quite tall. It varies in height but normally extends about 55,000 feet from the ground in our area.

Sometimes during fog events, the fog lasts because a temperature inversion has set up in the lowest levels of the troposphere.

Normally in the troposphere, air temperatures get cooler with height. But in a low-level inversion, the temperature is actually warmer than right at the surface.

On a typical day, as the sun warms the atmosphere, the fog slowly fades away as we begin separating the air temperature from the dew point.

In an inversion, things are different.

As the sun warms the atmosphere, the fog can’t dissipate until the surface temperature exceeds the temperature above.

This is called an inversion cap, and this cap “traps” the fog and keeps it in place.

The inversion prolonged the fog event Tuesday morning.

The process can sometimes take several hours, which is exactly what happened Tuesday morning.

The dense fog didn’t fade away until mid-morning, with some areas of fog in place until midday.

This weather pattern persists into Wednesday, and more fog is likely to develop overnight Tuesday into early Wednesday.

So the delayed sunshine Tuesday is a result of an inversion higher up in the atmosphere.

About the Author:

David Heckard is The Weather Authority's Assistant Chief Meteorologist.