coincident with the greatest temperature contrasts. Two of the most important frontal zones are those over the north Pacific and north Atlantic Oceans. In winter, the arctic front, a boundary between polar and arctic air, forms in high latitudes over northwest North America, the north Pacific, and near the Arctic Circle north of Europe (fig. 4-25). In summer, the arctic front mainly disappears, except north of Europe. (See fig. 4-26.) The  polar  front,  on  the  other  hand,  is  present  the year round, although it is not as intense in the summer as  in  the  winter  because  of  a  lessening  temperature contrast  between  the  opposing  air  masses.  The  polar front  forms  wherever  the  wind  flow  and  temperature contrast   is   favorable.   Usually   this   is   the   boundary between tropical and polar air, but it may form between maritime  polar  and  continental  polar  air.  It  also  may exist between modified polar air and a fresh outbreak of polar   air.   The   polar   front   is   common   over   North America  in  the  continental  regions  in  winter  in  the vicinity of 50°N latitude. The polar front in winter is found most frequently off the eastern coasts of continents in areas of 30° to 60° latitude.   It   is   also   found   over   land;   but   since   the temperature contrasts are greater between the continent and  the  oceans,  especially  in  winter,  the  coastal  areas are more favorable for formation and intensification of the polar front. The intertropical convergence zone (ITCZ), though not truly a front but a field of convergence between the opposing  trades,  forms  a  third  semipermanent  frontal type. This region shows a seasonal variation just as do the trade winds. FRONTAL CHARACTERISTICS LEARNING   OBJECTIVE:    Describe    the frontal elements and general characteristics of fronts. FRONTAL ELEMENTS From  our  previous  discussion  and  definitions  of fronts,  it  was  implied  that  a  certain  geometrical  and meteorological  consistency  must  exist  between  fronts 4-26 cA SOURCE MP 1020 1020 HIGH HIGH cA A M SOURCE 1000 MP M C P P 1020 HIGH MT CP (I C T   Z) MT MED 1020 C SOURCE T H CT 1020 HIGH L LOW 1020 HIGH MT 1010 C R MT 1020 HIGH MT CT CT LOW C SOURCE P 1032 C SOURCE A CA 180 150 120 90 60 30 0 30 60 90 120 150 180 75 60 45 30 15 0 15 30 45 60 75 60 45 30 15 0 15 30 45 60 DI JANUARY AG5f0425 Figure 4-25.—Chart showing world air masses, fronts and centers of major pressure systems in January.