Every clear night, a telescope on the island of La Palma looked at the sky and recorded how much the air dimmed starlight. This was routine work for astronomers measuring stellar positions. But in these records, like in an old diary, traces of a different story remained: the arrivals of dust from the Sahara.

Dust from the Sahara rises into the air and travels across the Atlantic. When it blankets the island, stars become fainter. Scientists took data from the Carlsberg Meridian Telescope from 1984 to 2012 and picked out nights when the light was attenuated especially strongly. They called these nights dust events.

It turned out that in summer, from July to September, dust arrives more often: in about 28% of clear nights. During the rest of the year, that figure is only 10%. Moreover, from 1984 to 1997, the frequency of dusty seasons dropped from 47% to 10%, then partially recovered and stayed around 26%.

Scientists also found that the upcoming dusty season can be predicted. If in winter the pressure difference between the Azores and Iceland (the North Atlantic Oscillation) was above or below normal, and if the Sahel received little rainfall in February through April, then there would be more dust over La Palma in summer. These two indicators explain more than half of the year-to-year variation in dust frequency.

Now astronomical archives can serve not only for studying distant galaxies, but also as a climate archive. The same telescope that measured stars turned out to be a sensitive probe of Earth's atmosphere. And its records allow a glimpse into the future of dusty seasons half a year ahead.