What is the defining characteristic of Ice Fog?

The defining characteristic of Ice Fog is that water vapor turns directly into ice crystals. This phenomenon occurs under specific conditions, primarily when the temperature dips significantly below freezing, usually in very cold environments. As the moist air cools, the water vapor condenses directly onto tiny particles, forming ice crystals instead of liquid water. This transformation can result in the visual effect known as fog, but instead of the typical water droplets seen in regular fog, the tiny ice crystals create a distinct appearance.

The other options provided do not align with the characteristics of Ice Fog; for example, it does not occur in high temperatures, nor is it a result of radiation cooling in the same sense as ground fog. Additionally, Ice Fog does not form due to heating from below; rather, it forms in the presence of cold air and existing water vapor. Understanding these conditions clarifies why the transformation of water vapor to ice crystals is the essence of Ice Fog.