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The interannual variability in the sea fog frequency (SFF) in July in the midlatitude Northwestern Pacific (40°N–50°N, 140°E–170°W) from 1979 to 2009 is investigated with observations and reanalysis datasets. Composite analysis shows that in high-SSF years the center of the Northwestern Pacific subtropical high (SH) shifts eastward and a strengthened ridge exists in the midlatitude Northwestern Pacific. Under such conditions, large amount of moisture from the subtropics are transported northwardly by the southerlies over the west flank of the SH. The ridge is helpful for stable stratification and conductive to fog formation. In contrast, in low-SFF years the center of the SH expands westward and drifts further south; thus moisture can hardly reach the midlatitudes. Meanwhile an anomalous trough in the midlatitudes and the associated anomalous northerlies both weaken the southerlies and reduce the stability, unfavorable for fog occurrence. The case studies confirmed that the air parcels moving from the subtropical zone to the midlatitudes controlled by the SH, kept the higher temperature and humidity when flowing across the Kuroshio Extension, and then cooled down over the cold oceanic surface in fog case. The SFF in the Northwestern Pacific would decline under the conditions of global warming. [ABSTRACT FROM AUTHOR]