Business men customarily wear coats and ties. Public officials tend to dress like business people. Neither men nor women wear shorts in public. Jeans and informal attire are fine for touring and shopping. San Pedro de Atacama is a tourist attraction, and dress is always casual.
Days are warm, and evenings are cool year round.
Take clothing that will let you survive the freezing nights if your vehicle breaks down. A ski parka or a down jacket, gloves, a hat with ear protection, and wind pants work well. Sun glasses are helpful. Drink lots of water. Use sunburn protection.