Fog forms when the difference between the air temperature and the dew point is less than 4.5°F or 2.5°C.

 Fog is a thick layer of water droplets suspended in the air near the ground. It occurs when warm air interacts with cold air, causing the water vapor to condense into liquid water. Fog is similar to a low-lying cloud, and is often difficult to see through. 

Here are some things to know about fog:

Formation

Fog forms when the difference between the air temperature and the dew point is less than 4.5°F or 2.5°C. 

Types

There are different types of fog, including radiation fog and valley fog. Radiation fog forms at night when the earth's surface cools, and valley fog forms when air from the mountains drains into the valley. 

Density

Fog is denser than mist, and reduces visibility to less than 1 km. 

Adjectives

Some adjectives that describe fog include patchy, swirling, persistent, thick, dense, and impenetrable. 

Factors

Fog is influenced by nearby bodies of water, topography, and weather conditions.