What does the abbreviation ONCL refer to in meteorological reports?

The abbreviation ONCL in meteorological reports stands for "Occasional isolated clouds." This term is used to indicate that there are clouds present in the sky but that they are not widespread or continuous. Instead, these clouds appear sporadically and are separated by clear skies. This descriptor is important for pilots and others in aviation, as it provides information on the visibility and potential weather conditions. Understanding cloud cover is crucial for flight operations; knowing that clouds are occasional and isolated can suggest fewer obstacles to visibility compared to more extensive cloud coverage.

The other options don't align with standard meteorological abbreviations or interpretations used in weather reports. "Overcast non-cumulus layers" is not a recognized cloud type, as "overcast" typically indicates a full cloud cover without isolating conditions. "Obstructed night cloud layers" and "overnight cloud levels" also do not correspond with established meteorological terms and may confuse the conditions being reported.