From December to April is summertime with hot and dry weather and temperatures ranging from 25° to 35°C. These are the best months for swimming.
Wintertime starts in May and lasts till November when the temperature drops a bit and it is cloudy. On the coast, the climate is determined by cold sea-water adjacent to deserts, and so the prevailing coastal winds absorb the cold Peruvian current moisture, which condenses only from May to November.
The resulting cover of clouds and sea mist extends from the south to about 200 km north of Lima. This so-called "garua" dampens isolated coastal zones of vegetation (called "lomas") and are grazed by livestock driven down from the mountains. During the "garua" season, only the northern beaches near Tumbes are warm enough for a pleasant swim.
From November to April is the rainy season, which is typical of drought and clear mostly in the morning, in the afternoon there is rainfall, with a small drop in temperature (18°C) and a small difference at night (15°C).
From April to October is the dry season, with hot days when the temperatures reach 20° - 25°C, and cold nights, often below freezing.
Peru's main season starts in June and lasts till August, which is the best time for hiking on the Inca Trail or for trekking and climbing elsewhere in the country. At this time, the days are generally bright and sunny, although the nights can be very cold at high altitudes. The highland can also be visited at other times of the year, although in the wettest months from November to April, some roads are inaccessible and hiking trails can be very muddy.
In the jungle areas in the south, a cold front can pass at night. November - April is the rainy season, whenever there can be heavy rains, humidity, and heat. During the season, it only rains for a few hours at a time, which is not enough to ruin your journey, but enough to make some roads virtually impassable.
April to October is the dry season having temperatures of up to 35°C. This is the best time to visit the jungle. In the jungle areas of the south, a cold front can pass through at night.