<P> Although the centre and the eastern parts of the country are mostly flat , the west is mountainous . Both the Andes and Sierras Pampeanas affect the climate of Argentina , leading to differences in temperature , pressure , and spatial distribution of precipitation depending on the topography and altitude . Here , the Andes exert an important influence on the climate . Owing to the higher altitudes of the Andes north of 40 S , they completely block the normal westerly flow , preventing low pressure systems containing moisture from the Pacific Ocean from coming in . Thus , much of Argentina north of 40 S is dominated by wind circulation patterns from the South Atlantic High . South of 40 S , the Andes are lower in altitude , allowing much of Patagonia to be dominated by westerly winds and air masses from the Pacific Ocean . However , the north -- south orientation of the Andes creates a barrier for humid air masses originating from the Pacific Ocean . This is because they force these air masses upwards , cooling adiabactically . Most of the moisture is dropped on the Chilean side , causing abundant precipitation and cloudiness while on the Argentine side , the air warms adiabatically , causing it to become drier as it descends . Thus , an extensive rain -- shadow is present in much of Patagonia , causing it to receive very little precipitation . The Sierras Pampeanas influences the climate on a much smaller scale than the Andes . </P>
where does the warm moist air mass over the andes mountains in argentina come from
the Pacific Ocean