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Southern Peru's forum places September in the shoulder season, still the dry season, with mild days in the 60s-70s°F and genuinely chilly nights in the highlands, sometimes dropping into the 30s-40s°F. Regulars flag blue skies as typical this time of year with little to no rain, though warm layers for the cold nights are essential regardless of how sunny the day was.

Regulars are specific this marks the true dry season here, essentially no rain expected across a typical trip, winter bringing colder nights that can drop into the low 30s Fahrenheit even as days still climb toward 70F. Our row backs that cool, genuinely bone-dry read, 23.2C over 9.6C nights feeling as low as 6C, with rain essentially absent and humidity down to just 37 percent. Regulars are consistent nearby Colca Canyon runs noticeably colder still, warm clothing genuinely necessary for that side trip regardless of how mild Arequipa itself feels. Layering to shed through a warm afternoon and add back for a cold evening remains the standard advice. The record: 10.3 hours of sun, a 10.9 kph wind and AQI 37.

Inland Peru in August is the dry winter and regulars describe it here as warm days around 16C with cool nights and no rain. That 16C is our monthly mean, not the afternoon: our row splits it into 23.5C by day and 8.9C at night, a fifteen-degree spread, and the night reads as 5C once the air is accounted for. It is extraordinarily dry air - 31 percent humidity, a dew point of minus 6.2C - and the rain column is a flat zero for the month. Layers are the answer, ideally ones that cut wind. At 2,330m this is also the gentle first night for anyone heading to Cusco, high enough to start on and low enough to sleep. The record: 10.7 hours of sun and AQI 33.

October sits just before the rains, which regulars date from December to March, and they put the month at 19 to 24C with a little cloud - our row gives 23.6C by day and 9.8C at night, a fourteen-degree swing at 42 percent humidity that is the high desert rather than the tropics. The altitude point matters more than the temperature: Arequipa is at 2,330m, while a Colca Canyon trip crosses 4,800m, so the cold they warn about belongs to the day trip and not the city, and an extra day or two here is the standard acclimatisation advice. For condors, eight to ten in the morning, when the thermals come up the canyon. May and September are the months they name for dry weather without crowds. The record: 10.5mm across 2.5 precipitation days, 11.5 hours of sun and AQI 35.

May opens Arequipa's dry winter - May through September - and regulars pick it over July and August for the same weather with fewer people in it. One May report on the forum describes rain most nights with almost entirely dry days - worth reading as a single year rather than the pattern, since our row has 1.2 precipitation days carrying 3mm across the whole month. What they insist on is the altitude, and the numbers make the argument. Arequipa sits just over 2,300m, which does little to prepare anyone for Cabanaconde at around 3,300m, and the bus to the Colca crosses above 4,900m getting there. Two nights in the city before the canyon is the standard advice rather than the cautious version of it. The record: 23.6C days against 10.4C nights that feel like 7C, 1.2 precipitation days carrying 3mm, humidity 50 percent and 10.5 hours of sun.