Let there be lightweights: that's the inside word on outerwear. The unexcelled comfort of the newly launched line of topcoasts, many in fabrics as light as suiting cloths -- airy tweeds, sleek sharkskins, rugged coverts, silk, wool and man-made blends -- proves that man can not only survive but thrive in a state of sartorial weightlessness from early fall through late spring. Lighter in tone as well as avoirdupois, the new coats for business and casel wear have abandoned the austerity of charcoal shades in favor of warmly muted medium grays, olives, browns and tans. (Black, of course, remains first choice for evening and formal excursions.) Patterns for business and casual wear include a wide selection of tastefully subdued herringbones, checks, plaids and overplaids. In style, the choice is choice indeed, ranging from split and full raglans to trim semifitted chesterfields, from the straight-hanging box profile to the modified Continental silhouette. The new topcoats are shorter and trimmer looking than in seasons past -- most are about knee level -- offering the active male a new freedom of movement and a jaunty look of updated traditionalism. These words to the weather-wise should be sufficient; now give the once-over -- lightly -- to our handsome coaterie of bantamweights. Far left: our man keeps smartly warm in lightweight single-breasted wool herringbone topcoat with weltedged shawl collar, 3/4 Continental cuffs, hacking flap pockets, center vent, by Barry Walt, $135; felt hat with offbeat British brim-roll sides, downturned front, narrow brim, center crease, by Thomas Begg, $15; coordinated gloves with imported buffed calfskin backs, triple-pointed, hand-stitched capeskin palms, notched wrists, by Countess Mara, $15.50. Left: our guy sets out for exurban weekend in tailored informality of warm but weightless imported cheviot fitted chesterfield coat with hacking pockets, hooked center vent, notched lapels, outside breast pocket, full lining, by Duncan Reed, $75; felt hat with narrow brim, center crease, wide band, by Knox, $20; whip-stitched pigskin gloves with elastic side walls, by American Astral, $6. Right: Met-bound metropolite is lightly, rightly garbed in sumptuous imported black cashmere topcoat with velvet collar and cuffs, white satin lining, rounded notch lapels, hacking flap pockets, centervent, by Malcolm Kenneth, $210; white crepe muffler with black and white hand-knotted fringe, by A. Sulka and Co., $20; capeskin formal gloves with snap closures, by A. Sulka and Co., $8.50; elegant gros-grain collapsible silk opera hat, by Churchill, $35.