Given the huge variety of landscapes and ecosystems, it is no surprise that northern Peru has a number of climatic features, which impact on when it is best to visit:
Pacific Coast & Equatorial Dry Forest: this region, that includes the cities of Trujillo, Chiclayo, Piura, and the northern beaches of Mancora, Organos and Vichayito, is very much a year-round destination.
The day-time temperature seldom drops below 25°C (79°F) and coastal breezes prevent the heat being stifling. The hottest 'summer' months are from December to April, during which time there is slightly more light precipitation.
Peruvian Andes: as you would imagine, this vast mountain range has a great variation of climatic conditions. Temperature is proportional to altitude, varying from temperate (annual average of 18°C / 64°F) in the low-lying valleys to frigid (annual average below 0°C / 32°F) at higher elevations.
The maximum temperature is often steady throughout the year; the minimum varies according to the presence of cloud cover, which helps maintain the daytime heat during the night. In the absence of clouds, nights are much colder.
The altitude of the two major northern Andean cities, Chachapoyas (2,435 m / 7,989 ft) and Cajamarca (2,650 m / 8,695 ft) is not that high - when compared with Cusco (3,250 m / 10,660 ft) or Lake Titicaca ( 3,810 m / 12,500 ft) in the south, for example - and accordingly temperatures are quite mild.
Even in Huaraz, located at just over 3,000 m / 10,000 ft and surrounded by beautiful, snow-capped mountains, temperatures seldom descend to freezing point.
Precipitation has a marked seasonality, which may influence the timing of your northern Peru holiday. The rainy season in the Peruvian Andes is considered to start in late October / early November, but generally peaks between January and March. As an indicator, Chachapoyas receives an average of nearly 800 mm / 32 inches of rain per year, with the months of February and March having the most rainfall.
The driest months correspond with the Northern Hemisphere summer, ie. June to August, but the absence of cloud cover can mean colder nights and mornings.
Snowfall is frequent above 5,000 m (16,404 ft) during the rainy season, and occasional above 3,800 m (12,467 ft) between May and August.
Amazon Basin: is rich, lush, and green because it receives an abundance of rain (4 m / 12 ft a year on average). In a typical year, that adds up to 200 rainy days, which means that there may be days of heavy rain, at any time of year.
It is also almost continuously hot, with average temperatures in and around Iquitos, ranging between 20°C (68°F) and 33°C (91°F), while Tarapoto, which is located on a high jungle plateau at 356 m (1,168 ft), is a little fresher.
Nonetheless, Peru's Amazon Rain Forest is known for having two seasons, mainly owing to it being the basin into which the rainfall and meltwater from the Andes drains, with dramatic effects:
The Flooded Season runs from December through April - summer and autumn in the Southern Hemisphere. It is a little cooler and wetter, but bear in mind that only a little more than half (60%) of the total rainfall falls during the flooded season ... and it is hot in the Amazon any time of year.
There are great benefits to traveling at this time of year. Perhaps most important, the rivers and streams are about 7 m (23 ft) higher, meaning that every tributary, creek and lake is navigable. You will get to explore more of the waterways of Amazonia, and will have access to plant life and wildlife areas that you might miss during the dry season. Ideal if you are on an Amazon cruise.
Also, that extra seven metres puts travellers much closer to the jungle canopy, where mammals such as monkeys play, and Amazon birds like to roost. And this is the time of year when the Pacaya-Samiria Reserve becomes the 'jungle of mirrors', with its black water flooding the forest and creating wonderful reflections.
Many rare fruits and flowers bloom during this season, bringing birds and other animals to the river's edge, allowing travelers to observe.
Increased navigability has the disadvantage that areas to walk are limited at this time, and such usable hiking trails as there are will have more mosquitoes.
Fishing is more limited during these months too, but you still have a good chance to make a catch.
The Low Water Season in Amazonia coincides with 'winter' in the Southern Hemisphere (from June to November). Temperatures are slightly lower, and despite it also being called the 'dry season', there are still some heavy rains.
With the drop in water level, the Amazon River's beaches are unveiled and jungle paths that were flooded from December to May are now easily accessible, allowing groups to explore deeper into the jungle (accompanied by fewer mosquitoes).
Caiman are much more visible, and the pink river dolphins are less likely to be spread out along the river.
During these months, the fishing is better, with a 99% guarantee to catch a piranha! And while the lower river levels mean that you are farther from the birds that roost in the jungle canopy, you will have the chance to see dozens of species of migratory birds in flight, something you would completely miss during the flooded season.
In conclusion, the Amazon in northern Peru is a year-round destination, but with different attractions and conditions over the course of the year.