Other than precipitation the three main things that can inhibit visibility are fog, mist, and haze. The international definition of fog is a visibility of less than 0.62 miles, or 1 kilometer.
Mist is a visibility of between 0.62 and 1.2 miles (1 to 2 kilometers), while haze is a visibility between 1.2 to 3.1 miles (2 to 5 kilometers).
Aside from the visibility categorization, fog and haze form in a totally different way. Mist and fog are caused by water droplets in the air, and the only difference is visibility, as discussed above. On the other hand, haze is the reflection of sunlight off of air pollution, typically found on hot and humid summer days.
Note: Mist is not the same thing as drizzle or light rain. Drizzle is precipitation consisting of numerous tiny droplets of water less than 0.02 inches in diameter. Light rain is simply rain falling at a rate of 0.1 inches or less per hour. Both drizzle and light rain fall to the ground, while mist and fog, which remain in the air close to the ground.