What causes the Pacific coast of Peru and Chile to have an arid climate?

The Pacific coast of Peru and Chile tends to be arid because of the cold ocean currents that come from the Southern Ocean around Antarctica.

Explanation:

A big portion of the western coast of South America, or rather the Pacific coast, tends to arid or semi-arid. This is most dominant in Peru's and Chile's coastal areas, where we actually have the Atacama, the driest desert in the world. The reason for this is the influence of the cold ocean currents.

The cold ocean currents from the Southern Ocean around Antarctica move along the Pacific coast of South America. With the water being cold, the humidity is very low, meaning that the air masses coming with the ocean currents are dry. Dry air masses don't result in precipitation when they start to accumulate as they come upon a natural barrier like the Andes, so a big portion of Peru's and Chile's coastline is very dry.