I think this is probably the airport cloud coverage reporting directly translated into numerical values
Different types of cloud cover conditions include SKC (sky clear), FEW (trace), SCT (scattered), BKN (broken) and OVC (overcast). Cloud cover is reported in terms of 1/8th of sky cover with 1-2/8th being FEW, 3-4/8ths being SCT, 5-7/8th being BKN and 8/8 denoted at OVC.
source: https://www.universalweather.com/blog/aviation-weather-tips-all-you-need-to-know-about-ceilings/
Here’s the total cloud coverage distribution for Jan 1, 2004 to Dec 31, 2018 for Washington DC:
