The climate varies from place to place in Chile, as the geography is incredibly varied. Also, in some regions of Chile, the seasons are not entirely predictable, so please take the following as a guide and be prepared for changes in climate.
Most people traveling to Chile will be heading to Patagonia at some point during their stay. Patagonia has weather like Alaska, with a mix of sun, rain and wind on almost any given day. From October to mid-April the temperatures vary from the 40s to mid-60s during the day, sometimes a bit higher for stretches when the sun stays out. At night it will typically be in the 30s but can drop down into the 20s. The weather in Patagonia is highly variable, and you should be prepared for strong winds and wet weather by having some type of shell or Gore-Tex over layer or other wind and rain-protecting jacket.
The Lakes District has a climate like the Pacific Northwest of the United States, but with the seasons reversed. There is a cool mountain climate from October to April, with temperatures ranging from the 40s and 50s at night to the 60s and 70s during the day. Though this is the drier part of the year, rain is still common, though it generally won’t last all day. You should be prepared for windy conditions as well. A good Gore-Tex shell or similar jacket is very good to have in the Lake District for wind and rain. The wet season is from May/June to October and it rains fairly predictably during these times, though there can be periods of better weather.
Santiago has a Mediterranean climate, with an average temperature in the low 80s in January and about 50 in July (the middle of their winter). The dry season is September to April and the wet season is May to August. Even during the summer months of late December through late March the temperature can drop down into the 50s in Santiago.
San Pedro de Atacama
In Atacama, the climate is generally hot and dry all year, although like any desert it can get cold at night. There are places in Atacama where there has never been any recorded rainfall; however, during the winter (June-July), there is occasional flooding in the region and the temperatures at night can sometimes drop into the 40s. If traveling for stargazing, January and February tend to be the cloudiest months of the year but are still clear more than 50% of the time. For the best stargazing experience, it is recommended to avoid traveling around the dates of a full moon.