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In Mexico, the area from the delta of the Colorado River to southern Colima (ca. 2,000 km) is a transition zone from the most arid deserts of North America to tropical rainforests. Data for temperature and precipitation from 254 meteorological stations, along with field data, were used to analyze relationships between climate and vegetation. Several bioclimatic classification schemes were contrasted to explain distribution of the main types of vegetation. Vegetation follows a spatial arrangement associated with a north-south rainfall gradient, i.e., hyperarid and arid shrub deserts in a tropical-desert bioclimate, semiarid thornscrubs or thornforests and dry summer-green deciduous forests in a tropical-xeric bioclimate, and semi-evergreen rainforests in a tropical-pluviseasonal bioclimate. This main gradient is strongly related to two moisture-defined zones: the northern subtropical high-pressure cell, which controls the zone of lesser rainfall with a tropical-desert bioclimate, and the trade wind belt, which causes most rainfall in rainier zones. This zonal pattern can, however, be substantially modified by local factors, mainly mountain rainshadows.