What condition is necessary for ice fog to form?

Ice fog forms under conditions of very low temperatures, typically much below freezing. In this state, water vapor in the air can directly crystallize into ice particles, leading to the formation of fog that consists of tiny ice crystals rather than liquid water droplets. This phenomenon occurs because the air's capacity to hold moisture diminishes significantly at colder temperatures, causing the water vapor to condense and freeze.

In contrast, options involving warm or moist conditions or temperatures above freezing are not conducive to ice fog as those conditions generally lead to the formation of liquid fog instead. Similarly, dry air movement would reduce humidity levels, making it less likely for the moisture needed for ice fog to be present. Thus, the presence of extremely cold temperatures is essential for the formation of ice fog.