What characteristic defines upslope fog?

Upslope fog is specifically characterized by its formation due to moisture-laden air being forced upward along rising terrain, such as hills or mountains. As the air ascends, it cools and condenses, forming fog. This process is directly related to topographical influences and is not dependent on time of day or season.

The other options fail to address the fundamental nature of upslope fog. For instance, while fog can occur under various wind conditions, upslope fog is particularly associated with the lifting of moist air, making high winds irrelevant to its definition. Additionally, fog can occur during any time of day and in different seasons, depending on environmental conditions, meaning it is not limited to nighttime or summer months. Understanding that the unique mechanism of air lifting and cooling defines upslope fog helps clarify why the second choice is the correct answer.