If any part of South America approaches the four-season regime familiar to Americans and Europeans, it's the heart of Argentina. Most of the country gets recognizably warm in summer and cool in winter, with clear-cut transitions in between. What's more, the bulk of Argentina manages a great deal of sunshine without becoming a desert. Some spots average 100 clear days a year and 8 hours of sun a day. When it does rain, it's often in the form of showers or thunderstorms that sweep through quickly.
Much of Argentina's population is clustered along the Parana River and on the Rio de la Plata, which snakes its way close to the country's eastern border, reaching the Atlantic just past Buenos Aires. These rivers are a useful meteorological boundary. Eastward, toward Uruguay and eastern Paraguay, rainfall and humidity increase. Buenos Aires itself is fairly sultry in midsummer, though much less oppressive than Rio de Janeiro. To the north is the Gran Chaco, a scrubby lowland that extends into northwest Paraguay. It's the hottest spot in South America: parts of the Gran Chaco reach 45°C/113°F almost every summer (winters can be quite cool, though).
West and south of the Parana lies the heartland of Argentina and its endless fields of treeless pampas. Sunshine rules during the summer, with hot days (sometimes scorchingly so) and mild nights. On many afternoons, thunderstorms form just off the Andes and move eastward. These can be quite intense – large hail is a perennial threat, especially in and around the Sierras de Cordoba – but rainfall is seldom extreme. Especially in spring, temperatures may soar across the pampas courtesy of the zonda, a dusty fohn-type wind. Winters average on the cool side, but temperatures can gyrate from well below freezing to readings above 26°C/79°F. The highlands of the Puna de Atacama experience equally dramatic temperature swings, but shifted toward the cold side; above 4000m/13,000ft, it's unlikely to exceed 20°C/68°F any time of year. Lightning-laced storms dot the Puna de Atacama in summer; they're less common as you move south along the eastern Andes.
Summer afternoons are quite warm across the lowlands as far south as the Rio Negro, but once you enter Patagonia the averages begin to drop and the clouds and wind become more frequent. Winters tend to be fairly cloudy across all of Patagonia. Summers bring periods of sunshine across the north, but it's actually the cloudiest, gustiest season across the extreme south. The Argentinian part of Patagonia is far drier than its Chilean counterpart: many spots qualify as desert, with less than 200mm/4in of rain or melted snow each year. Although summer is the dampest time overall, any month can bring some moisture. There's a bit more temperature range here than on the Chilean side. As far south as Sarmiento, many summer afternoons top 26°C/79°F, while most winter nights fall below freezing. It's colder as you move south or gain elevation: some protected valleys above 4000m/13,000ft have been known to dip below –20°C/–4°F.