If an element does not occur or cannot be observed, it is classified as:

When an element does not occur or cannot be observed, the appropriate classification is to omit it from the report. This means that if a specific weather element, such as rainfall or cloud cover, is not present or cannot be measured, it is simply not included in the documentation of observations. This is important for maintaining clarity in the reports, ensuring that only visible and measurable elements are recorded. Omitting such elements helps avoid any potential confusion regarding the state of the weather at that time.

In contrast, other classifications such as marking an element as "optional" would lead to ambiguity about whether it should be included or not. Additionally, noting it as "NA" (not applicable) can suggest a different context, typically indicating that the condition does not pertain to the situation rather than that it simply cannot be measured. Similarly, excluding it from all records is excessive, as relevant observations should still be captured even if specific elements are missing. Therefore, omitting is the most accurate and practical approach for unobserved elements.