Published online 6 January 2000 | Nature | doi:10.1038/news000106-11

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Stars and spuds

For many centuries Andean farmers have been looking to the stars to predict rainfall for the growing season. There may be more to this whimsical behaviour than meets the eye, reports Philip Ball.

Weather folklore is as old as legend, and as unreliable. "Take care not to sow in a north wind or to graft and inoculate when the wind is in the south," said the Roman writer Pliny, in an adage the precise opposite of one that circulated in the seventeenth century. But it is not all hot air and guesswork. US scientists report in Nature1that a traditional method of forecasting rainfall used for centuries by farmers of the Andes gains some support from modern meteorology.

Many weather proverbs claim to make predictions for one season based on the weather in an earlier season. "If the spring is cold and wet, then the autumn will hot and dry," says one. But the farmers in some communities of the Peruvian and Bolivian Andes stretch plausibility still further by using the stars to forecast the weather. They believe that the rainfall during the growing season of October to May can be predicted from the brightness of stars in the Pleiades constellation in June. The brighter the stars, they say, the more abundant the rains.

The observations of the constellation-both its apparent brightness and the time of its first appearance in the pre-dawn sky-are afforded considerable importance in the Andean culture. And well they might be, for an accurate prediction of rainfall is crucial to a successful harvest of potatoes, the main crop in the region. Potatoes are very vulnerable to drought. So if poor rains are predicted, villagers delay planting for several weeks after the usual time in October and November. That way, the plants are more likely to benefit from later rains.

The tradition of observing the Pleiades is many centuries old, and it is hard to imagine how it could have lasted so long if its predictions were useless. The Pleiades constellation in fact holds a central position in Andean belief: it was worshipped by the Incas, and used by the Mayan priests to calculate the calendar on which planting times were based. But why should the stars have anything to do with rain?

Benjamin Orlove of the University of California at Davis and co-workers wondered whether the visibility of the constellation, which also determines the time of its first appearance in the sky, might be related to the presence of wispy cirrus clouds high in the atmosphere. These clouds would veil the starlight without being directly visible themselves. They hypothesized that the presence of such wispy clouds might be connected with a climate pattern that also affected rainfall during the Andean growing season.

The climate of the west coast of central and south America is strongly influenced by El Niño events, which recur every two to seven years. El Niño events are part of a natural oscillation of the world's climate system called the El Niño-Southern Oscillation (ENSO), the cause of which is still imperfectly understood. The ENSO cycle has a 'warm' and a 'cold' phase. The warm, El Niño phase manifests as an anomalous warming of surface waters in the equatorial Pacific Ocean, which generates unusual weather in many parts of the world: wetter in some regions, drier in others.

One of the known effects of the ENSO warm phase on the weather of Central and South America is that it increases the amount of cirrus cloud cover over the Andes. Orlove and colleagues have shown that this can reduce the apparent brightness of the Pleiades sufficiently to be noticable even with the naked eye. They have also examined rainfall records between 1962 and 1988, and find that there is less rain during ENSO warm years, and that the rainy season in those years begins later than October. Thus, it seems that by examining the stars in June, the Andean farmers are obtaining a crude gauge of the likelihood that the ENSO cycle will deprive them of rain later in the year. 

  • References

    1. Orlove,B. S., Chiang, J. C. H. & Cane, M. A. Forecasting Andean rainfall and crop yield from the influence of El Niño on Pleiades visibility Nature 403, 68 2000. | Article | PubMed | ISI | ChemPort |