Incan Lore Rings True
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In the central Andes of South America, a single harvest can make the difference between feast and famine. Farmers from the ancient days of the Incan empire, to now, have "read" the sky to figure out the best time to plant their crops. Recently, this ancient custom has been validated by science. Hi I'm Bryan Yeaton and this is The Weather Notebook.

It's during this time of year that winter begins in South America. Each night during the middle of June, mountain villages across Peru and Bolivia closely watch the northeast sky for the Pleiades constellation. The Incans worshiped this cluster of stars. If the Pleiades were dimmer than usual, that was a bad sign. They learned that there would be less rain than usual for the growing season to come. If the stars shone brightly, that meant abundant rain.

Why would the stars go dim before a drought? It turns out that high, thin cirrus clouds often sweep across the Andes in the summer before an El Niño kicks in. These clouds may be too thin to see with the naked eye, but they're enough to cause a noticeable dimming of the stars at night. The villagers of long ago correctly figured out that cirrus was a sign of the drought that El Niño would later bring to their land, even though they didn't know anything about El Niño itself.

Villagers across the Andes still watch the stars in June. If the stars are dim, they'll delay their potato planting a few weeks so that the dry spring is less likely to damage their crop.

The Weather Notebook is a production of the Mount Washington Observatory, with support from the National Science Foundation.