Peru's climate varies considerably by region and altitude. The best time for trekking is June to September, when days are sunny but nights are cold. The wettest months are January through March (when the Inca trail is often closed). The coast is quite hot and humid during those months, becoming cooler during the rest of the year by La Garùa mist blown in from the cold ocean. As one moves up into the mountains, night-time temperatures become considerably colder. The eastern slope of the Andes, like the Amazon basin, experiences very heavy rainfall during the wet season, so best time to visit would be April to October.

Autumn: Temperature approx. 15°C. Begins in May.
Winter: Temperature approx. 13°C. Begins in June.
Spring: Temperature approx. 17°C. Begins in late September.
Summer: Temperature approx. 24°C. Begins in December.

It should be remembered that Peru has local festivals throughout the year and at certain times like Carnival, one should book up a long time ahead.

See also:

Peru - When to go: Climate charts