Clouds play a key role in climate by reflecting solar radiation and by trapping heat from Earth. Understanding how clouds form is critical to modeling climate effects. Studies reveal that airborne mineral dust and metal particles are essential for forming cirrus clouds, which are wispy and found in high altitudes (Science 2013, DOI: 10.1126/science.1234145). A group led by Daniel J. Cziczo of Massachusetts Institute of Technology studied cirrus ice crystals during four aircraft measurement campaigns from 2002 to 2011. The researchers found that most of the particles in the air near clouds are made of sulfate and organic carbon. They also found that in the cloud formation process, ice preferentially condenses on aluminosilicate dust or metal particles. The results point to a heterogeneous ice nucleation mechanism in clouds, akin to getting a compound to crystallize from solution by scratching the beaker, rather than a homogeneous mechanism in which ice nucleates without a substrate.