How does fog form?
Fog is reduced visibility caused by tiny water droplets or ice particles suspended near the ground. It can be understood as a ground-level cloud. Air reaching saturation is central, but several different processes can produce it.
Radiation, advection, and evaporation fog
On clear calm nights, ground cooling can chill nearby air and produce radiation fog. Moist air moving over a colder surface can form advection fog. Adding vapor from warm water to cold air can also generate fog. Cold-air pooling, wind, and topography matter, so fog is not limited to one season or time of day.
Dissipation and visibility
Surface heating, increased mixing, or incoming dry air can dissipate fog, although advection fog may persist during daylight. High relative humidity alone does not determine visibility; droplet size and concentration also matter. Mist, fog, and air pollution are distinct but can coexist. For transportation, current local visibility measurements may be more informative than a general weather summary.