Forecasting Andean rainfall and crop yield from
the
influence of El Nino on Pleiades visibility
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 crop(1). Here we use data on cloud cover and water 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 usually linked to reduced
rainfall during the growing season several months later. Our results suggest that this
centuries-old method of seasonal rainfall forecasting may be based on a simple indicator of El Nino
variability.
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