Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Weather; Procedure; Human Factors
Preflighted aircraft at gate in SEA during light drizzle and 4C OAT.  I inspected the wing for possible clear ice after fueling.  I did not detect any ice; and passengers were boarded.  Take off and climb out were normal with anti-ice on.  In flight; cabin crew called us to inform us that they saw ice formations on the top of the right wing.  The formations were streaked in the slipstream like rivers of ice.  There were no unusual airframe or aerodynamic vibrations.  Flight was continued with no further problem.
B737 First Officer reports inspecting the wings for ice formation after refueling during light drizzle and finds no ice. The Flight Attendant's believe they see ice  on the wings during flight but no abnormal flying qualities are noted.