Well, water if you are talking about natural fog.
Ice and fluid if you are talking about a machine
It depends mainly on the visibility - how far you can see.
Water vapor
Fog is made of tiny drops of water
The fog lamp cover is custom made. But, you can make one very easily.
Fog can be made thicker by the presence of smoke, as the particles in the smoke can serve as condensation nuclei for water droplets to form around. This can result in the fog becoming more opaque and reducing visibility even further.
Water vapors in the air
no sht shrlock
Fog looks like cloud but only on the ground not the sky.Like clouds ! Clouds and fog are both made up of tiny water-droplets, fog is just cloud that's close to the ground.
Mostly warm and sunny, but sometimes cold and wet. Fog is common, but more often smog.
Low beams. Better yet, if you have them, use fog lamps that are mounted low on the vehicle and are aimed mostly down at the road.
Mexico is mostly made of desserts.
Newfoundland. They have 90-mph fog there.