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For at least the past four centuries, indigenous potato farmers of the Peruvian and Bolivian Andes have gathered in midwinter to gaze up into the night sky and observe the Pleiades. If this star cluster appears big and bright to them, they think that they will have plentiful rains and big harvests the next summer; if the cluster appears small and dim, they anticipate less abundance. Their belief is so strong that they time the planting of their crops accordingly. One might imagine that this practice amounts to nothing more than an odd superstition, but it turns out that this scheme actually works: The apparent size and brightness of the Pleiades varies with the amount of thin, high cloud at the top of the troposphere, which in turn reflects the severity of El Niño conditions over the Pacific. Because rainfall in this region is generally sparse in El Niño years, this simple method provides a valuable forecast, one that is as good or better than any long-term prediction based on computer modeling of the ocean and atmosphere.
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