Fog

Piloting a drone in fog is strongly advised against. First of all, it means risking losing the UAV. Second of all, the UAV can sustain damage which might be costly or impossible to fix. However, sometimes manual or autonomous flight in fog is a necessary evil.  That's why some training missions come with fog as an environmental condition. Thus, future pilots can learn to fly a UAV in fog and to make difficult decisions based solely on their own skills.