Fog and cloud present very little difference in respect of their appearance and structure, because both of them are a visible aggregate of microscopically small water droplets floating in the atmosphere. The only difference between them is that while the fog forms at or near the ground, clouds form at much higher altitudes.
image source: scijinks.jpl.nasa.gov/review/fog/1280px-December_Fog_01.jpg
In fact, the essential difference between these two condensation forms lies in the method and places of formation rather than in structure and appearance. Clouds are formed when air rises, expands and cools adiabatically.
Fog, on the other hand, forms as a result of radiation cooling or the movement of air over a cold surface. Fog may also form through cooling of the air by contact and mixing or through saturation of the air by increasing its water vapour content.
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Fog is defined as almost microscopically small drops of water condensed from and suspended in air near the surface of the earth in sufficient number to reduce the horizontal visibility to less than one kilometer.
Fog is also defined as a cloud with its base at or very near the ground. A light fog may reduce the visibility to 2 or 3 kilometers, but in case of a dense fog visibility is reduced to a few meters.
Fog occurs during the calm or light wind conditions. It is more common in the vicinity of ocean where there is an abundant supply of moisture near the earth’s surface than inland. In most localities fog is more common during winter than summer. It is relatively uncommon in the tropical lands.