In general, the spring and summer seasons in the Southern Hemisphere (October-March) are characterized by long days when weather is warmer and drier. The peak tourist season when accommodations and air travel arrangements can be most limited, including holiday (Christmas and New Year) and summer vacation for locals in January and February.
Climate in Patagonia is similar to the Pacific Northwest and southern Alaska; in the summer you can expect some warm sunny weather, as well as cold, rainy, windy days. Geographical diversity throughout the rest of Chile and Argentina gives rise to several distinct climatic regions with wide extremes of temperature, sunlight and rainfall. Although the season for adventure travel in Patagonia is limited, you can travel in many areas of northern Chile and Argentina throughout the year. The prime season in Patagonia is October-March when average daytime highs range from 65° F in the sun to the low 40°s. Night temperatures are typically in the mid to upper 30°s, but occasionally in the 20°s. The weather is extremely erratic in Patagonia. It varies from warm sunshine to drizzle, to rain and/or sleet in a matter of minutes, and returning to sunshine just as quickly. Wind is a constant factor, ranging from strong breezes of 15-20 mph to gale forces up to 60 mph. Clothing must be appropriate, adaptable outdoor gear.
Climate in Buenos Aires is temperate and moderated by maritime air. Expect warm, sunny days during summer (70-80°F in Dec-Mar) and cool, cloudy weather in winter (50°F, June-August). The Andean region, including the Lakes District from Puerto Montt to Bariloche, has a cool mountain climate from October-April (50-55°F). The tropical environment of Iguazu Falls is great to visit year round. Expect high humidity and temperatures from mid to high 70°s in Jan-Feb. The high Andean region of northern Chile and Argentina (Salta-Juyjuy Provinces) is typically dry and clear, especially in May-September.