The Long Foggy Night

Fog is a rather common weather phenomenon but do you know exactly what it is and the science behind it? Did you know that there are different types of fog? Hopefully after reading this article, you’ll have a better understanding of fog.

What is Fog?

The atmosphere is filled with water attracting particles called hygroscopic nuclei. When water vapor in the atmosphere begins to cool and condense onto these hygroscopic nuclei, the relative humidity increases and the visibility in the atmosphere decreases because of the grayish tint that is exhibited. These water collecting hygroscopic nuclei have water droplets on them and they then start to become so large that they are visible with the naked eye. When the visibility drops to 2/3 of a mile and the air is wet with millions of tiny floating water droplets, the wet haze becomes a cloud resting near the ground and we call it fog.

Fog, like any cloud, usually forms in one of two ways: 1 By cooling, the air is cooled to the point where it becomes saturated with water vapor, we call this the dew point or 2, by evaporation and mixing. Water vapor is added to the air by evaporation and the moist air mixes with relatively dry air. Once fog forms, it must keep its degree of saturation either by continual cooling or by evaporation and mixing of water vapor into the atmosphere.


Let’s take a quick look at one of the most common types of fog.

Radiation Fog (Ground Fog)

This type of fog is common in eastern North Carolina. It forms best on clear nights when a shallow layer of moist air near the ground is trapped by a layer of dry air over top of it. Since this layer is really shallow, it doesn’t absorb a lot of the Earth’s outgoing radiation so the ground cools rapidly so does the air above it. We call this an inversion because the air directly above this layer is drier and warmer. This moist lower layer quickly becomes saturated to the dew point temperature and fog forms. The longer the night, the longer the time of cooling and the greater the likelihood of fog forming. So radiation fog is most common over land in the late fall and winter because of the long nights. Fog is densest of thickest just before sunrise. A light breeze is also crucial to this type of fog as it brings more moist air in contact with the cold ground and the transfer of heat occurs more quickly. A strong breeze would allow the moist and dry air to mix, preventing fog from forming.

Fog will dissipate or “burn off” by mid to late morning (and in some cases the afternoon) as sunlight penetrates the fog and warms the ground, causing the air temperature in contact with the ground to increase. The warm air rises and mixes with the foggy air above, which increases the temperature of the foggy air. As some fog evaporates, more sunlight hits the ground and produces more heating and soon the fog completely disappears.


In upcoming blogs, we’ll take a dive into some of the other common types of fog.



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