The term 'become overcast' can be described by four different crossword solutions: DARKEN (6 letters), grayed (6 letters), GRAY (4 letters), and slaty (5 letters). DARKEN means to make something darker in color or tone, while grayed means to become GRAY or to make something GRAY. GRAY is a color that is a mix of black and white, and slaty is a shade of GRAY that is slightly bluish. Therefore, the term 'become overcast' can be described as becoming darker, grayer, or more bluish-GRAY in color or tone.