Buenos Aires and the Pampas are temperate; cold in the winter, hot and humid in the summer.
The deserts of Cuyo are extremely hot and dry in the summer and moderately cold and dry in the winter. Spring and fall often exhibit rapid temperature reversals; several days of extremely hot weather may be followed by several days of cold weather, then back to extremely hot.
The Andes are cool in the summer and very cold in the winter, varying according to altitude.
Patagonia is cool in the summer and cold in the winter. Extreme temperature shifts within a single day are even more common here; pack a variety of clothes and dress in layers.
Don't forget that seasons are reversed from those of the Northern Hemisphere.