Forecasting Andean rainfall and crop yield from the influence of El Nino on Pleiades visibility

Farmers in drought-prone regions of Andean South America have historically made observations of changes in the apparent brightness of stars in the Ple iades around the time of the southern winter solstice in order to forecast interannual variations in summer rainfall and in autumn harvests. They mode rate the effect of reduced rainfall by adjusting the planting dates of pota toes, their most important crop(1). Here we use data on cloud cover and wat er vapour from satellite imagery, agronomic data from the Andean altiplano and an index of El Nino variability to analyse this forecasting method. We find that poor visibility of the Pleiades in June-caused by an increase in subvisual high cirrus clouds-is indicative of an El Nino year, which is usu ally linked to reduced rainfall during the growing season several months la ter. Our results suggest that this centuries-old method? of seasonal rainfa ll forecasting may be based on a simple indicator of El Nino variability.