Farmers in the high Andes Mountains may have been able to predict El Nino for centuries by observing whether nearly invisible clouds dimmed the light from a cluster of stars called the Pleiades.

A tradition for potato farming handed down from one generation to the next teaches that the brightness of stars in the constellation during June roughly predicted the rainfall during the growing season from October to May.

The brighter the stars, the more abundant the rain for a potato crop highly vulnerable to drought. If poor rains are predicted, villagers in the mountains of Peru and Bolivia delay planting for several weeks.

Benjamin Orlove at the University of California at Davis and co-workers report Thursday in the journal Nature that modern meteorology supports the farming tradition.

His study of satellite and weather data suggests that wispy cirrus clouds high in the atmosphere are more prevalent during El Nino, a warming of the Pacific Ocean that occurs roughly every two to seven years.

``What is remarkable is how detailed this cloud data is,'' Orlove said.

Orlove, who spent more than 3 1/2 years in the Peruvian Andes, said the clouds obscure the dimmest five of the 11 main stars that form the Pleiades. The dimmer stars are on the outer edges of the constellation, so it appears to shrink when viewed with the naked eye, he said.

The changes apparently only can be seen in the clear, dry air of the high Andes, which have a unique weather pattern, Orlove said.

The farming tradition may extend back to the 15th century and the Inca empire, said Alan Kolata, a University of Chicago anthropologist who specializes in the Andes. He said the Incas, who unified the central Andes tribes, had extensive astronomical knowledge even though they lacked a written language.

``I believe it's a quite provocative, interesting work and Ben is certainly on to something,'' Kolata said.