2-18. The most troublesome type of fog to mariners is advection fog. Advection means horizontal movement. It is also the name given to fog produced by air in motion. This type of fog is formed when warm air is transported over colder land or water surfaces. The greater the difference between the air temperature and the ocean temperature, the deeper and denser the fog. Unlike radiation fog, advection fog is little affected by sunlight. It can and does occur during either the day or night. Advection fog is best dispersed by an increase in wind velocity or change in wind direction.