In most cases, fog is most likely to form, and most likely to be at its peak density during the early morning hours. The reason for this is that, following the normal or 'diurnal’ temperatures swings that take place during a 24 hour period, temperatures are at their lowest in the hour or so before dawn. Fog is more or less a cloud layer based at the surface, forming when the lower levels of the atmosphere reach 'saturation’. A less jargon filled way to say this; when the temperature reaches the dew point. And when the temperature reaches the dew point, under the right conditions, fog will form.

In China, fog is considered heavy when the horizontal visibility is less than 500m and more than 200m. Fog is considered dense when the horizontal visibility is less than 200m and more than 50m, and it is considered very dense when the horizontal visibility is less than 50 meters.

According to the U.S. National Weather Service, fog or smoke are considered dense when the visibility is lowered to a quarter of a mile or less. The combination of smoke and fog is a very dangerous situation that can lower visibility to zero. If dense fog is predicted or observed over a large enough area, the National Weather Service will issue a Dense Fog Advisory. For marine interests, the potential for dense fog will mentioned in the Coastal Waters Forecast, and in Marine Weather Statements as needed.