Total obscuration occurs when what proportion of the sky is obscured by surface-based obscuration?

Total obscuration is defined as a situation where the entire sky is obscured by surface-based obscuration, such as fog, smoke, or other phenomena that hinder visibility. In the context of visual assessments of sky condition, the sky is divided into an eighths scale, known as "eighths of the sky." Therefore, total obscuration is indicated when the entire sky, or 8 out of 8 eighths, is obscured.

This understanding is essential for pilots and meteorologists as it affects visibility significantly and influences flight operations and safety. In scenarios of partial obscuration, such as 2/8, 4/8, or 6/8, there would still be some visibility of the sky, allowing for weather observations and potential flight navigation. Thus, the concept of total obscuration specifically refers to the complete blockage, making the correct answer where 8 out of 8 portions are obscured.