Atacama Climate: How El Niño Runs the Telescopes

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The Atacama Desert in northern Chile is the driest non-polar place on Earth. The sky is almost always clear, and the air is so devoid of moisture that even distant galaxies are visible without interference. That is why radio telescopes, which are most affected by water vapor, stand on the Chajnantor plateau at an altitude of five kilometers. But even in such an ideal location, there is a "weather conductor": the Pacific El Niño cycle. It alternately warms and cools the ocean, and with the water temperature, observing conditions change.

Astronomers from the European Southern Observatory set out to determine exactly how the phases of this cycle affect the telescopes. They gathered data from four sites: La Silla (2400 m), Paranal (2650 m), and two points on Chajnantor (5080 and 5150 m). Temperature and humidity records from weather stations, measurements of water vapor, and image blurring by turbulence (astronomical seeing) were compared with the Niño3.4 index, which is the deviation of ocean surface temperature in the tropical Pacific from the average. If the deviation is greater than 0.75 degrees, it is called El Niño; if less, La Niña.

At the same time, astronomical seeing itself, the blurring of a star's image by turbulence, remains stable. The median value of about 0.7 arcseconds does not change with the years or between El Niño and La Niña phases. Only during strong La Niña events does the spread of seeing increase somewhat, making conditions less predictable. In other words, El Niño controls humidity and temperature but does not touch turbulence, like a conductor who sets the rhythm of dryness and warmth but does not interfere with the strings.

This work gives astronomers the first practical tool for long-term planning. Previously, they could only rely on seasonal patterns; now El Niño phases can be used as a predictor of dry and warm conditions. This is especially important for the future Extremely Large Telescope (ELT) with a 39-meter mirror, which is being built near Paranal. If you know in advance that El Niño is coming, you can schedule complex infrared programs for that period instead of wasting telescope time due to unsuitable humidity.

So El Niño is not just a weather phenomenon; it is a kind of schedule for telescopes. Warm phases bring dryness and heat, cold phases bring moisture and coolness, but the image quality itself remains almost unchanged. Astronomers can now align their plans with this Pacific rhythm, just as musicians follow their conductor. And although the conductor is not always predictable, his score is becoming clearer.