1. Department of Environmental Science and Policy, University of California, Davis, California 95616, USA
2. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA

Correspondence to: Benjamin S. Orlove^1,2 Correspondence and requests for materials should be addressed to B.S.O. (e-mail: Email: bsorlove@ucdavis.edu).

Abstract

Farmers in drought-prone regions of Andean South America have historically made observations of changes in the apparent brightness of stars in the Pleiades around the time of the southern winter solstice in order to forecast interannual variations in summer rainfall and in autumn harvests. They moderate the effect of reduced rainfall by adjusting the planting dates of potatoes, their most important crop¹. Here we use data on cloud cover and water vapour from satellite imagery, agronomic data from the Andean altiplano and an index of El Niño 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 Niño year, which is usually linked to reduced rainfall during the growing season several months later. Our results suggest that this centuries-old method² of seasonal rainfall forecasting may be based on a simple indicator of El Niño variability.