Santiago sits close to the middle of Chile, about 60 miles inland from the Pacific Ocean. Peak tourist season falls during its summertime—from December to March—when temperatures sway between the mid-50s and mid-80s. That's also, however, when the city's smog seems most apparent.
If you'd prefer a quieter (and perhaps cleaner) scene, the shoulder seasons in Santiago are lovely. The city sees far less tourists in the spring (September to December) and in the fall (March to May), and the weather is pleasant. In the spring, temperatures hover between the high 40s and low 70s, while in the fall, things get a bit chillier. Average temperatures in April are in the 60s.
For skiers and winter-sports fans, the winter—from July to September—can also be a great time to visit, especially in August when there's already a good base of snow on the mountains. Santiago is an easy jumping off point for world-class ski resorts in the surrounding Andes, including Portillo, Valle Nevado, and El Colorado/Farellones. Keep in mind, though, that the city itself gets very little snowfall. The white stuff generally falls only at elevations above 4,900 feet.
Down in Santiago, which sits between 1,312 and 1,772 feet, winter usually means chilly rain.