Daily synoptic analyses for five winter and summer seasons are used to obtain mean frontal positions for the Northern Hemisphere. These positions are found to differ in several important respects from earlier estimates based on mean-pressure and -temperature charts.

In winter, three axes of high frontal frequency, or principal frontal zones, appear which are termed the Pacific polar front, the Atlantic polar front, and the Eurasian polar front. There is also some evidence of a weak Atlantic arctic front. A fourth belt of high frontal frequency parallels the Rocky Mountains.

In summer, four principal frontal zones are delineated: a Pacific polar front, an Atlantic polar front, an Eurasian polar front, and a Siberian-Canadian arctic front.

The locations and intensities of the oceanic fronts are shown to be closely related to the sea-surface temperature distributions. The formation of a Siberian-Canadian arctic front in summer is attributed to the strong thermal contrast that develops along the borders of the arctic seas. The position of the Eurasian polar front in winter also appears to be affected by the thermal characteristics of the underlying surface, being most distinct where open seas adjoin relatively cold land.

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1 This research has been sponsored by the Geophysical Research Directorate of the Air Force Cambridge Research Center under Contract AF 19(604)-3063.

2 Contribution No. 52, Dept. of Meteorology and Climatology.