After mornings like today, ever wonder what causes fog? While there are at least four different types of fog, caused by different weather and geographic phenomenon, there are two types that were showing up in our viewing area this morning: radiational fog and sea fog. 

The fog we experienced was influenced by a few different factors: clear skies, very light wind (2-5 mph) and high dewpoints (over water primarily). When you have clear skies, the air can cool quickly, creating a small dewpoint depression (difference between the temperature and dewpoint). The dew point temperature is the temp that a parcel of air needs to be cooled to for it to become saturated. If the dewpoint depression is less than 2 degrees, than you’re more likely to get fog.

When you have dry air aloft, meaning up high typically where clouds form, it cools quickly. Dry air is more dense than moist air, meaning a shallow layer of moist air at the surface that you get when the air cools (especially over a moist ground) hangs and is trapped by the denser air above it.

When winds are very light, but not calm, it keeps the ground close to the saturation point – rather than drying it. This helps aid in fog formation.

In terms of sea fog, it is more common over colder water. The Gulf water temperature – about 15 miles from shore to inland – is typically a few degrees cooler than the open waters. When wind comes in from the Southeast, South or Southwest, it’s pushing air that was over warmer water over colder water. The air just above the water is very close to the saturation temperature, or dewpoint, and when it moves over cooler waters, it tends to cool to the air and condense it into fog. The temperature of the air and the water over the warmer water cannot be largely different, because it will only cool a by a few degrees when it moves over cooler waters.

Fog burns off faster over land than water because water holds heat much better than land. When the sun beats down through the fog, it heats the ground, and releases heat from the ground that eventually overcomes the fog. Many people believe fog is burned off from the top down, but it actually “lifts” from the surface up.

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