Abstract: A heat exchanger is provided with a set of oppositely disposed header pipes into which a plurality of parallel cross flow tubes are inserted and joined to provide fluid communication between the header pipes. Fluid enters the header pipes and flows through the tubes, wherein it is cooled by air passing over the tubes. The tubes are constructed from two separate plates, each plate having a plurality of non-continuous ridges, which are joined together to form a single substantially flat tube having an upper wall and a lower wall. The ridges are oriented along each wall of the tube in substantially oblique configurations, relative to the longitudinal axis of the tube, so that upstream ends of the ridges in each configuration are aligned in a substantially oblique line which extends across the width of the tube.