Peru’s coastal desert sees a seasonal low cloud called garúa, cold, persistent fog pushed inland by ocean currents. Where that fog hits hillsides, moisture is available even where rain is virtually absent. Fog catchers are simply vertical meshes hung on frames where the fog flows; tiny droplets collide with the filaments, coalesce into larger drops, run down gutters, and are stored in tanks for later use. These systems are passive, low-energy, and relatively cheap to install.