The cold Pacific or Humboldt Current is the reason why Peru's coast is so dry. Moisture evaporating from the ocean is absorbed by the warmer air over land, keeping clouds from forming. In winter, however, when the air temperature drops slightly, heavy fog, called garua, forms on the coast and drifts inland against the western foothills of the Andes Mountains. On some of these hills, especially those that face south, the direction of the prevailing winds, the mist condenses on rocky outcrops and tree leaves and drips onto the ground. By September, these hills, called lomas, are covered with green vegetation and blooming flowers. Some of these lomas are home to endemic bird species (those found nowhere else on Earth) and other wildlife takes advantage of the abundance of food during the winter and spring. The lomas are under threat, as cities and farms expand and people cut trees for fuel. One of these areas, the Lomas de Lachay, north of Lima, has been set aside as a natural reserve.

Scientists believe the lomas around Lima, the Peruvian capital, probably once had heavier vegetation than they do now. After the Spaniards established the city, trees were cut for fuel and building materials. Some scientists have installed fog catchers to trap droplets of garua and channel them into storage tanks. The water can be used to water trees and other plants, restoring the natural ecosystem. Fog catchers are wire mesh panels mounted on frames that face the prevailing wind. Droplets condense on the mesh and trickle into channels.