What is the condition for sky cover to be considered variable?

Sky cover is classified as "variable" when it changes across one or more reportable values in terms of cloud coverage. This means that observations show different amounts of cloud cover at different times or locations within a given area. For example, a report might indicate that the sky cover is changing frequently from scattered clouds to broken clouds. In this context, reportable values refer to specific categories of coverage such as clear, scattered, broken, or overcast.

The other options do not accurately capture the concept of variable sky cover. While having more than one layer of clouds or being affected by windy conditions might lead to changes in visibility or other weather phenomena, these factors alone do not specifically define sky cover as variable. The critical aspect is the fluctuation in the cloud coverage categories that can be reported, which is why the identification of varying reportable values is central to understanding the variable condition.