and  low  pressure  over  the  Equatorial  Belt  sets  up  a pressure gradient directed from north to south. Because of the flow around the massive Siberian high, northeast winds begin to dominate the regions from India to the Philippines. (See fig. 3-23). During the winter months, clear skies predominate over  most  of  the  region.  This  is  caused  by  the  mass motion of air from a high-pressure area over land to an area of lower pressure over the ocean. As the air leaves the high-pressure area over land, it is cold and dry. As it travels over land toward the ocean, there is no source of moisture   to   induce   precipitation.   The   air   is   also traveling  from  a  higher  altitude  to  a  lower  altitude; consequently, this downslope motion causes the air to be  warmed  at  the  adiabatic  lapse  rate.  This  warming process has a still further clearing effect on the skies. During  the  summer  the  airflow  over  the  region  is completely   reversed.   The   large   interior   of   Asia   is heated to the point where the continent is much warmer than   the   ocean   areas   to   the   south.   This   induces relatively  low  pressure  over  Asia  and  higher  pressure over  the  equatorial  region.  This  situation  produces  a southwesterly flow as shown in figure 3-24. 3-21 COLD SIBERIAN HIGH EQUATOR 1014 1024 1024 1020- 1018 1016 1014 10 10 1012 1012 1018 AG5f0323 40^O O O O O O O O O 50 60 70 80 90 100 110 120 WARM OCEAN SURFACE Figure 3-23.—Northeast monsoon (January). AG5f0324 40^O O O O O O O O O 50 60 70 80 90 100 110 120 HOT INTERIOR COOLER OCEAN SURFACE 1008 1012 1010 1000 1004 99 98 1000 1004 1006 1010 1012 EQUATOR 1008 1006 1000 Figure 3-24.—Southwest monsoon (July).