How does fog form?

On some mornings, the world outside seems to disappear into a soft gray haze. Familiar streets and hills fade away, and visibility shrinks to just a short distance. This is fog, and while it can feel mysterious, it is actually a very ordinary weather event. In fact, the simplest way to understand fog is this: fog is essentially a cloud that has formed right down at the ground, instead of high up in the sky.

The air around us always contains some water in an invisible gas form called water vapor. Warm air can hold a large amount of this water vapor, while cool air can hold much less. As long as the water stays as vapor, we cannot see it and the air looks clear. The amount of water vapor the air can hold before it becomes full, or saturated, depends heavily on its temperature. This relationship between temperature and moisture is the key to fog.

Fog forms when the air near the ground cools down. As air cools, its ability to hold water vapor drops. If the air cools enough, it reaches a point where it can no longer hold all its water vapor as a gas. At that point, the water vapor begins to condense, changing from invisible gas into tiny visible droplets of liquid water. These droplets form around microscopic particles floating in the air, such as dust or salt. A great number of these tiny droplets hanging in the air together is what we see as fog.

There are several common ways the necessary cooling happens. On clear, calm nights, the ground loses heat and cools the air just above it, which can produce a low-lying fog by morning, often settling in valleys. Fog can also form when warm, moist air drifts over a colder surface, such as cold ground or cool ocean water, and gets chilled from below. In each case the basic recipe is the same: moist air is cooled until its water vapor condenses. As the sun rises and warms the air again, fog often fades away, since the warmer air can once more hold its moisture as invisible vapor.

This article was written using information from Wikipedia.