Fog along the southern coast of Peru can be present at any time of the year. However, I have found the least fog problems in the Peruvian summer of Jan/Feb/March/April. I have encountered the worst fog in November and December - scary, pull off the road type of fog. Other times of the year it just depends, but chances are fog could be a problem in October. I have never made the entire drive from Arequipa to Lima, nor would I want to - long, slow and a lot of truck/bus traffic.