Fog is tiny water drops hovering in the air. It is like a cloud on the ground. Thick fog makes it difficult to see the surrounding landscape.
Fog forms from water vapor, which turns back into liquid, when the air cools. A gentle wind helps fog to form and to stay in the air. It is very common in valleys and near bodies of water and usually forms at night. In the morning it disappears.
Water droplets in the fog have a size of only about 0.01 mm. A thick fog contains approximately 1200 drops per cubic cm.