When to visit Peru?

I asked my friend Romina and this was her reply:
"November to March is rainy only in the mountains, that time of the year is perfect to visit the coast. You can visit Machu Picchu by bus whenever you want, but it may be a little bit rainy on your way there. The perfect time to come to Peru and travel is April-May and September-October."

So what are you waiting for?

Interestingly, the Estudio Nacional del Fenómeno El Niño (ENFEN) is predicting this will be another El Nino year. If you forgot about the 1997-98 El Nino (or if you are a big nerd like I am), I highly recommend you read this National Geographic article. Lima's climate is generally quite mild and dry yet the relative humidity is very high so every morning there is a brief fog in the summer and persistent low clouds from May to November. It is cloudy, damp and cool in the winter (Jun-Oct) and sunny, moist and warm in the summer (Dec-Apr). Inland locations within Lima receive around 2 inches of rainfall, mostly in the winter months. Summer rain is mostly isolated light and brief afternoon showers. The Andes are a different story. The peak of the 'rainy season' however, name which really does not apply, occurs during winter when late-night/morning drizzle events (locally called 'garua', 'llovizna' or 'camanchaca') become frequent. All these climatic phenomena arise from the combination of semi-permanent coastal upwelling and the presence of the cold Humboldt Current just offshore. A complete breakdown of the weather can be found here: Undercovertourist.