Case studies of tropical-extratropical air mass interactions over South America are presented in this module. Synoptic features associated with cold air intrusion into tropical latitudes and the incursion of humid, tropical air masses into midlatitudes are identified. Conceptual models illustrate the synoptic environments and the role of the Andes in the meridional movement of air masses. Forecast challenges are presented for different periods of each case. Finally, the weather impacts associated with these contrasting air-mass incursions are explored.