What phenomenon occurs when water vapor turns directly into ice crystals in very cold temperatures?

The phenomenon that occurs when water vapor turns directly into ice crystals in very cold temperatures is known as ice fog. This happens when the air is significantly below freezing and contains enough water vapor, causing the vapor to deposit onto surfaces as ice without first becoming liquid water. Ice fog is often seen in extremely cold environments where humidity is present, and the resulting ice crystals can create a visually striking effect as they hang in the air or settle on surfaces.

In contrast, rime fog involves water vapor freezing on contact with surfaces, creating frost but not necessarily hanging in the air densely like ice fog. Steam fog occurs when warm, moist air comes into contact with cold surfaces, leading to the condensation of water droplets; this does not involve transition to ice directly. Mixed fog refers to a combination of different types of fog, often including moisture from varying sources, making it less specific than the definition of ice fog.