Yamase Air-Flow (YAF) is an outflow of the summertime maritime polar airmass over the North Pacific toward the San-Riku district of Japan (SRJ) and forms along the southern or southeastern edge of an anticyclone developed over the Okhotsk Sea (Okhotsk Sea high). In most summers, the Okhotsk Sea high appears intermittently, but the high and associated YAF persisted for an abnormally long period between mid July and mid August in 1993. Due to the continuous YAF, an abnormal low temperature accompanied with synoptic-scale (several or ten days) variation was observed in the SRJ during this period. We studied mechanisms of the AirMass Transformation (AMT) of the YAF in 1993 over the western North Pacific for both relatively warm and cool spells of the variation.<br>The temperature of the YAF at the coast of the SRJ varied over a narrow range around the offshore Sea Surface Temperature (SST). The temperature sometimes exceeded the SST plus 3°C only for a short period in the warm spells but never fell below the SST minus 3°C in the cool ones. Namely, the offshore SST controlled the lower limit of the YAF's temperature. The temperature variation was related to trajectories of air-parcels in the YAF over the western North Pacific. The YAF was directed southwestward toward the SRJ from Kuril Islands during the cool spells, while directed southwestward over the North Pacific and turned northwestward off the SRJ during the warm spells. Since SST gradient is large off the SRJ, the meridional direction of the YAF largely affected the AMT of the YAF. Trajectories of the YAF were related to the southeastward extension of the Okhotsk Sea high into the North Pacific, which was stronger in the warm spells.<br>During the cool spells, an atmospheric mixed layer accompanied with low-level clouds appeared in the YAF at the coast of the SRJ. The ocean supplied sensible heat of ∼30Wm^-2 and latent heat of ∼80Wm^-2 to the YAF on its way from the neighborhood of Kuril Islands to the SRJ. This heating compensated strong radiative cooling at the top of the low-level clouds (∼-70Wm^-2) and kept the temperature of the YAF higher than the offshore SST minus 3°C.<br>During the warm spells, a stable layer attached to the sea surface accompanied with low-level clouds appeared in the YAF. Off the SRJ, the temperature of the YAF exceeded the offshore SST by several degrees and the YAF was cooled (-10∼-20Wm^-2) and lost its moisture (0∼-20Wm^-2) at its bottom. The low-level clouds also contributed to the cooling of the YAF through the radiative process (∼-70Wm^-2). The cooling of the YAF on its way to the SRJ, however, sometimes may not have been enough to adjust its temperature to the SST when northwestward advection of the YAF off the SRJ was rapid or the radiative cooling was weakened by upper-level clouds.<br>Even during the cool spells, the AMT of the YAF was much weaker than that of the Asian winter monsoon breaking out over the Kuroshio area. In the AMT of the winter monsoon, radiative cooling at the top of the low-level clouds can be neglected in comparison with the dominant heat supply from the ocean. In the AMT of the YAF, on the other hand, the radiative cooling was comparable to the oceanic heating or cooling.