What is Fog? - Definition, Types & Causes

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  • 0:04 A Spooky Night
  • 0:39 What Is Fog?
  • 1:21 Radiation Fog
  • 1:43 Advection Fog
  • 2:41 Fog in Cold Climates
  • 3:32 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
In this lesson, we'll talk about fog - what it is and why it happens. Fog comes in many types, and here we'll cover the definition of each and how they are formed.

A Spooky Night

Picture an old-fashioned horror movie. You're driving down a one-lane road deep in the woods with your sweetie on Halloween. You're headed to a party across town. Your headlights are on, but it's hard to see because there is a haze in the air. Suddenly, your car starts to sputter and stop. You pull over and start to inspect your car, but all of a sudden, something jumps out of the shadows! But you can't see who or what due to the heavy fog.

This lesson discusses this classic horror-film feature known as fog. Fog occurs outside of horror movies as well, and outside of the woods, too!

What Is Fog?

Although it might feel a bit spooky, fog is actually just condensed water vapor close to the ground. To understand fog, we first need to tackle humidity. The air around us can hold a certain amount of water vapor, or water in a gaseous state. As more and more water fills the air, the air feels more humid. The amount of water vapor in the air is known as humidity. When the water vapor completely saturates the air, the water droplets start to condense, or turn from a gas back into a liquid. These droplets of liquid are suspended in the air and appear as a thick haze, known as fog. The types of fog are separated into three main categories, and some of these categories have multiple types.

Radiation Fog

Radiation fog is fog that is due to the cooling of the earth's surface at night. In the evening, the earth radiates off heat absorbed from the Sun's light during the day. As the warm air rises, the air near the earth's surface becomes cooler. Cold air can hold less water vapor than warm air, and the water vapor in the air near the surface condenses, forming radiation fog.

Advection Fog

Advection fog is the fog that appears due to cool air mixing with warm air. Warm, moist air flows into a cooler area. Since cold air can hold less water vapor than warm air, the water vapor condenses and forms fog. This type of fog is common on the Pacific Coast of North America, where warm air from the Pacific Ocean mixes with cooler air on land.

Advection fog has several subcategories. Upslope fog is when warm air is forced up over a mountain. The warm air interacts with the cooler air, allowing water to condense and form fog. A variant of upslope fog occurs in mountain valleys, where the fog condenses and lingers in the mountain valley, called valley fog.

Another type of advection fog is when warm raindrops fall into an area of cooler temperatures, called frontal fog. Steam fog occurs over oceans, where the cold air from the ocean mixes with warm air above. This causes fog to form above the ocean surface.

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