Peru’s weather is a complex set of patterns caused by the Humboldt Current, the Andes, and the jet stream, which blows northwest (not southwest as in the Northern Hemisphere) and picks up the Amazon’s moisture as it travels.
On the coast, the southwesterly trade winds that blow toward Peru are chilled as they pass over the frigid waters of the Humboldt Current. When these cold winds hit Peru’s sun-baked coast, they gradually warm and rarely release rain because their ability to hold water increases. Clouds form only when the air begins to rise over the Andes, dropping rain over Peru’s mountain valleys and high grasslands.
During much of the year, however, a curious temperature inversion occurs along much of Peru’s coast. The rising air from the coast is trapped beneath the warm air over 1,000 meters, which is never cooled by ocean breezes. Fog blankets sections of the coast, and drizzle, known in Lima as garúa, falls lightly—mainly, but not only, between April and September.
The jet stream heads west over the Amazon basin  toward the Andes, picking up transpiration from the Amazon basin. As this warm, humid air rises over the Andes it also condenses into a fine mist that nourishes the cloud forest. As it rises even higher over the puna, it falls as rain.
The heaviest periods of rain in the Peruvian Amazon  and Andes occur between December and April, a time that Peruvians refer to as the época de lluvia (the rainy season). The first rains, however, begin in October, which is the beginning of the highland planting season. Soon after, rain from the Andes begins to cascade down on the coast, where desert farmers use it for irrigation, and into the Amazon, where rivers become swollen and muddy. The Amazon River can easily rise a staggering 7–14 meters during the brunt of the rainy season in the Andes, which is why most natives live in stilted homes.