Looking at a map, Santiago de Chile looks vaguely like Sacramento, California — a big city in a long north-south agricultural valley, with a low mountain range separating it from the Pacific and a high, steep cordillera to the east. Both are at about 35 degrees (one north, one south) so there is a persistent high pressure system over them all summer: air that rose in ecuatorial thunderstorms and dried out in the upper atmosphere falls back to earth at about 35 degrees, with the aridity especially notable around 28-38 degrees on the west coasts of all the continents.
But there are big differences. Maybe the biggest is that in Sacramento, despite more cars and plenty of industry, you can breathe the air pretty dependably. A few indications of why: