Vertical visibility is shown on Terminal Aerodrome Forecasts (TAF) reports when the sky is what?

Vertical visibility is included in Terminal Aerodrome Forecasts (TAF) when the sky is obscured, meaning that the horizon cannot be seen due to weather conditions such as fog, heavy rain, or snow. This indicates that, despite the presence of significant weather, the cloud layer may be present at a low height, making it impossible to see through to the horizon. This should alert pilots that flight conditions are complicated due to possible low visibility under the cloud layer.

In contrast, overcast conditions, partially obscured skies, or clear skies imply that there is either some degree of visibility or that the sky is not obscured at all, thus they do not require the reporting of vertical visibility in TAFs. Vertical visibility is specifically important for ensuring safety and effective piloting in situations where low visibility might threaten operations.