What is the primary characteristic of steam fog?

Steam fog is primarily characterized by the process of air saturation, which occurs when cold air moves over warmer water bodies, such as lakes or rivers. This leads to the creation of steam-like clouds that form close to the surface, resembling fog. The moisture from the warm water evaporates and saturates the cooler air, causing condensation and the formation of steam fog.

While it is true that steam fog often appears as a low-lying cloud (which connects to the first option), the essence of steam fog lies in the saturation of the air, which is the critical factor prompting its formation. The description of how steam fog typically lifts before noon due to solar heating reflects its transient nature but isn't the defining characteristic. Lastly, while steam fog may suggest certain weather conditions, it does not necessarily indicate a forecast of clear and cold weather, as other factors can affect visibility and temperature. Thus, option B captures the fundamental process behind steam fog formation, highlighting the main characteristic that distinguishes it from other types of fog.