A cooling fan assembly for facilitating the transfer of heat through a heat exchanger, the cooling fan assembly comprising: a fan (110; 210; 300) rotatable about an axis of rotation (116; 216) and defining a downstream direction along the axis of rotation (116; 216), wherein the fan (110; 210; 300) is positionable downstream of the heat exchanger, the fan (110; 210; 300) further including a plurality of fan blades (112; 212); and a fan shroud (100; 200) including: a barrel (118; 218; 306) concentric with the axis of rotation (116; 216) and encircling the plurality of fan blades (112; 212), the barrel (118; 218; 306) defining a barrel radius (124; 224) and having a base portion (122; 222); and a plenum (130; 230) including a plenum body (132; 232; 304) and at least one skirt (140; 240), characterized in the plenum body (132; 232; 304) coupled to the barrel (118; 218; 306) and having a substantially rectangular planform viewed in the upstream direction, the plenum body (132; 232; 304) defining an effective surface monotonically increasing in upstream distance from the base portion (122; 222), the at least one skirt (140; 240) extending substantially parallel to the axis of rotation (116; 216) and coupled to the plenum body (132; 232; 304), the at least one skirt (140; 240) including a substantially rectilinear upstream border having a first end (136; 236) and a second end (138; 238), and wherein the plenum (130; 230) defines for the at least one skirt (140; 240): a cross section C through the axis of rotation (116; 216) and normal to the skirt (140; 240); a distance D1 between cross section C and the first end (136; 236) of the upstream border, and a distance D2 between cross section C and the second end (138; 238) of the upstream border; a distance D3, wherein D3 is equal to the lesser of D1 and D2, or is equal to D1 if D1 equals D2; a plane P1 parallel to and offset from cross section C an offset distance DP1 less than D3; a plane P2 parallel to and offset from cross section C on the opposite side of cross section C from plane P1 an offset distance DP2 less than D3; a plane P3 perpendicular to both cross section C and the axis of rotation (116; 216) and containing a point on the upstream border; a plane P4 perpendicular to plane P3 and cross section C and containing a point located on cross section C and on the plenum body; a plane P5 parallel to and offset from cross section C an offset distance equal to D3; and a plane P6 parallel to and offset from cross section C on the opposite side of cross section C from plane P5 an offset distance equal to D3, wherein a volume V1 is defined as enclosed by planes P1, P2, P3, P4 and the effective surface, a volume V2 is defined as enclosed by planes P1, P3, P4, P5 and the effective surface, a volume V3 is defined as enclosed by planes P2, P3, P4, P6 and the effective surface, an average cross sectional area A1 is defined as V1/(DP1+DP2), an average cross sectional area A2 is defined as (V2+V3)/((D3-DP1)+(D3-DP2), and wherein A1 is less than A2. a fan (110; 210; 300) rotatable about an axis of rotation (116; 216) and defining a downstream direction along the axis of rotation (116; 216), wherein the fan (110; 210; 300) is positionable downstream of the heat exchanger, the fan (110; 210; 300) further including a plurality of fan blades (112; 212); and a fan shroud (100; 200) including: a barrel (118; 218; 306) concentric with the axis of rotation (116; 216) and encircling the plurality of fan blades (112; 212), the barrel (118; 218; 306) defining a barrel radius (124; 224) and having a base portion (122; 222); and a plenum (130; 230) including a plenum body (132; 232; 304) and at least one skirt (140; 240), a barrel (118; 218; 306) concentric with the axis of rotation (116; 216) and encircling the plurality of fan blades (112; 212), the barrel (118; 218; 306) defining a barrel radius (124; 224) and having a base portion (122; 222); and a plenum (130; 230) including a plenum body (132; 232; 304) and at least one skirt (140; 240),