Fog: Cause, Conditions, and Types

Fog facts

Fog is a weather condition that occurs in a variety of settings for a number of reasons. But the basics of fog are the same throughout these varied categories: fog is simply droplets of water vapor suspended in the air near the ground. More clearly stated, fog is a cloud that is in contact with the ground. Fog is what happens when we get up close and personal with clouds. Ever sat on the ground on a summer day and wondered what it would be like to be up amongst those fluffy clouds? Well, fog gives you that opportunity.

Fog Formation

Just the same as clouds in the sky, fog forms when the air becomes supersaturated. Supersaturated air is air that can no longer hold moisture in the form of vapor. When this happens, waterfalls out of the air, forming a fine mist of water droplets. If the water becomes dense enough, the droplets will turn into rain. This is why the air feels clammy when you walk through fog, the air is dense with water. When air is cooled to the point at which it can no longer hold the water, we call this the “dewpoint.”

The warmer air is, the more moisture it can hold in the form of vapor. Think, warm, and humid. Therefore, when air is cooled, its ability to hold water is altered. This means that when air is rapidly cooled fog can form easily. You may notice foggy air is commonly cooler than the neighboring air.

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Types of Fog

One common type of fog is “radiation fog.” Radiation fog forms typically after dark. The Earth begins to radiate heat outwards which rises into the atmosphere. As it rises from the Earth’s crust, it is cooled, causing fog. This type of fog generally dissipates as soon as the Sun warms the area in the morning.

Another type of common fog, “advection fog,” occurs when warm air blows across a cold surface such as ice, water, or snow. This is sometimes referred to as “sea fog.”

“Tule fog” is a weather phenomenon that occurs when colder air from high altitudes sinks into a valley at night. Warmer air above traps this cool air creating foggy conditions.


Some fog which is common to large cities is actually not fog at all, but smog. More frankly put, pollution. In this scenario, the air is holding particulate matter instead of water. Clearly, this can have significant ramifications for visibility as well as air quality.A description of the conditions necessary for fog formation and common types of fog.

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