For centuries, Andean farmers in Peru and Bolivia have monitored the brightness of Pleiades stars to determine when to plant potato crops for optimal rainfall. Now, scientists have validated the farmers' weather forecasting methods. They can demonstrate that atmospheric fluctuations -- in particular El Nino -- explain the variability in star brightness. El Nino is a weather phenomenon that occurs when trade winds that normally blow westward across the tropical Pacific Ocean relax, and the warm water in the western Pacific drifts back toward Peru and up the coasts of North and South America, prompting atmospheric changes. In a letter to the journal Nature published today, UC Davis environmental scientist Benjamin S. Orlove, and two colleagues at the Lamont-Doherty Earth Observatory of Columbia University, describe the reasons for the changing brightness using satellite data on cloud amounts and water vapor. Using the data, the scientists link poor visibility of the Pleiades in June, reduced rainfall during the growing season and lower harvests to El Nino warm events. When the Pleiades cluster is particularly bright or when it appears to be large, Andean villagers anticipate earlier and more abundant rains and larger harvests, while the opposite is linked with less clear skies. If poor rains are predicted, the farmers postpone potato planting, say Orlove and his colleagues. Using the satellite data, the researchers found that thin clouds above the earth's surface occur more frequently in El Nino years than in La Nina years. These additional clouds in El Nino years block some of the light from the Pleiades. This shift is sufficient to visibly reduce the apparent brightness of the Pleiades in El Nino years. "In particular, dimmer stars warn observers of a later start to the rainy season and reductions in overall rainfall and harvests. Armed with this forecast, indigenous farmers alter their cropping strategy by planting at a later date."