Stratus appear when warm air is rising in layers instead of bubbles.
This produces wide cloud bases because a stretch of air is reaching the condensation point at the same time. They are generally rather thin, under 10,000 feet (3,048 meters) in the air.
Fog is a stratus cloud that hovers at surface level.
Rolling fog comes from a mass of warmer air moving horizontally along a cooler surface, such as a pond or lake, or land that has been cooled overnight.