Let’s see, it’s the middle of winter in South America and at 12,000 feet altitude I’d be more shocked if it *wasn’t* cold.
They are only in to their third week of winter. -18C = -O.4F. Rather cold for that early in their winter, even at that altitude range. For but one web site providing information on the diverse climate of Peru including some stats on the mountain ranges,
Check Here. Notice for the past few years their winters have been trending to the colder side.