For a country fronting a tropical ocean, it's amazing that coastal Peru is as dry as it is. Some towns along the Pacific go years without measurable rain. The desert air, shaped by cold waters moving north along the coast and welling up from below, tends to be humid, yet mild, for its tropical latitude. From May to October, the sun is almost continually hidden behind a deck of low clouds that produce garua. This persistent but extremely light drizzle is most common at night and early morning. It falls a little more heavily where the clouds meet the Andes, at elevations around 500– 1000m/1650–3300ft. The result is a belt of greenery where, for at least part of the year, cattle can graze. During the high-sun months, clouds part and the air turns warm and humid – yet it still doesn't rain, apart from one or two showers each summer that stray west from the Andes. Note that El Nino can produce uncharacteristic downpours in January and February across northern parts of Peru, even where it may not have rained all decade; flood waters may pour over barren streambeds.
As you move south through Peru, the desert extends higher along the western Andes slopes. In the highlands themselves, it thunders about every two or three afternoons from September into April, but it's almost completely dry in June and July. Temperatures cool only gradually with height at first, with most of the elevation's influence coming in nighttime lows. Freezes in July are rare below 2500m/8200ft, but frequent above 3500m/11,500ft. At the latter height, it can just as easily snow as rain, even in summer (the snow seldom lasts more than a few hours, except on the very highest peaks). The Andes gradually descend into Peru's two eastern flanks: the Madre de Dios of the southeast, where rains are focused in the October-to-May wet season, and the Amazon headwaters of the northeast, where afternoon deluges occur almost every day throughout the year. There's only a slight easing of the Amazonian downpours around July, when the monthly average rainfall at Iquitos is at its lowest: close to 150mm/6in.