Ethnoclimatology in the Andes: a cross-disciplinary study uncovers a scientific basis for the scheme Andean potato farmers traditionally use to predict the coming rains.

Across the Andes in Peru and Bolivia, farmers gather in small groups in the middle of the night in late June. They climb high ridges and often ascend to the peaks of mountains. Coming right after the winter solstice, these nights are the longest of the year and among the coldest as well. Hundreds of such groups of villagers assemble on these nights in a large area that extends from Huancayo, located some 12 degrees south of the equator, to Potosi, which lies at 19 degrees south. The farmers huddle together in eager expectancy. They are waiting for the moment when they can see the Pleiades, a star cluster in the constellation Taurus.

At this time of year, the Pleiades become visible low in the northeast sky only as dawn nears. The farmers believe that they can use the particular appearance of the Pleiades to forecast the timing and quantity of precipitation that will fall in the rainy season, months later. Although this odd form of astrology might seem just a quaint superstition (like the Groundhog's Day ritual familiar to Americans), our research has, in fact, uncovered its scientific basis.

Our project began after two of us (Orlove and Cane) independently heard of these forecasts. Orlove first came across them in 1973, while conducting field research in the southern Peruvian Andes for his doctoral dissertation in anthropology. Curious to witness the yearly event, he arranged to join a group of indigenous farmers who gathered atop the nearest large mountain to await the appearance of the Pleiades above the horizon. He wrote an article about this practice, which is part of the festival of San Juan, celebrated each June 24th. But he focused on the social mechanisms that led people to form the groups and didn't consider the accuracy of the forecasts themselves. And once that article was finished, he dropped the topic. His notes from that first field work remained buried in a drawer.

Cane learned about this Andean ritual much later, while on vacation in Peru in 1994. On a hike with his wife--less than 150 kilometers from Orlove's field site--he struck up a conversation with the local guide about weather and climate. The guide mentioned the forecasts to him. They tickled his curiosity. It seemed to him, a specialist in climatology, that there might be some scientific basis to them. He took detailed notes, including the name of the Pleiades in Quechua, the indigenous language. After his return home, he raised the subject from time to time with people who he thought would be interested. One day in 1996, a graduate student in anthropology whom Cane knew suggested that he might discuss the matter with Ben Orlove--a not-so-common name that Cane recognized immediately. As it happens, the two had grown up six blocks from each other in Brooklyn and attended the same schools. Although they had been in regular touch as children and as teenagers, they had not seen each other in a quarter-century

An exchange of email messages ensued, and as they began to discuss the stories they had heard in South America, they found that they shared a common set of reactions. On the one hand, it seemed completely extraordinary. How could the appearance of stars possibly be connected to rainfall? And how, indeed, could people even remember the appearance of stars from one year to the next? Their belief, and the agricultural practices connected to it, seemed as implausible as foretelling the outcome of a battle by examining the intestines of a sacrificed bull. On the other hand, it wasn't impossible. There are many areas in which indigenous knowledge of this sort has shown its worth. Aspirin and quinine, for example, were once no more than folk remedies. Agronomists often turn to peasant farmers for their knowledge of local crop varieties. And in many parts of the world, architects are adopting the traditional building styles of desert peoples in recognition that these designs represent energy-efficient solutions for arid climates. If some cases of traditional knowledge have a sound footing in medicine, agriculture and architecture, there might be such instances in atmospheric science as well.