Answer:
A dense fog advisory has been issued in the region.
A choropleth map (from Greek χῶρος "area/region" and πλῆθος "multitude") is a type of thematic map in which areas are shaded or patterned in proportion to a statistical variable that represents an aggregate summary of a geographic characteristic within each area, such as population density or per-capita income.
When it was collapsed in 1989