An open top refrigerated display case having an ambient air defrost system and a method of operating such display case, at least one air conduit extends in a U-shaped formation along opposing side walls and a bottom wall of the refrigerated display case. Each of the air conduits has an opening at both ends with such openings being located at upper portions of the side walls and in alignment so as to direct air across the access opening for recirculation through the air conduit during a refrigeration cycle of operation. Arranged within the primary air conduit is a set of refrigeration coils and at least one reversible fan for circulating air through such air conduit. During a refrigeration cycle of operation, the fan circulates air through the air conduit in a first direction through the refrigeration coils. During a defrost cycle of operation, the fan circulates air through the air conduit in the opposite direction and draws in ambient air from outside of the display case. During such defrost cycle of operation, the air leaving the air conduit is directed and supplied with a sufficient momentum so that such air travels along a path over the opposing side wall of the display case so that the air does not reenter the air conduit.