Contrails in the blue sky remind us of daily air traffic – and its impact on the climate. However, the effect of contrails on the climate is still only partially understood. It is assumed that they have a predominantly warming effect. Researchers from Forschungszentrum Jülich and universities in Mainz, Cologne, and Wuppertal have now discovered: 80 per cent of all long-lived contrails do not form in cloudless skies, but within existing natural ice clouds, known as cirrus clouds. The climate impact of these embedded contrails has hardly been investigated to date. However, the study published in the journal Nature Communications provides new insights and could influence the planning of climate-optimized flight routes in the future.