At any given time, cirrus clouds（卷云） -- the thin wisps of vapor that trail across the sky -- cover nearly one-third of the globe. These clouds coalesce（使联合） in the upper layers of the troposphere（对流层）, often more than 10 miles above the Earth's surface. Cirrus clouds influence global climate, cooling the planet by reflecting incoming solar radiation and warming it by trapping outgoing heat. Understanding the mechanisms by which these clouds form may help scientists better predict future climate patterns.

Now an interdisciplinary team from MIT, the National Oceanic and Atmospheric Administration (NOAA), and elsewhere has identified the major seeds on which cirrus clouds form. The team sampled cirrus clouds using instruments aboard high-altitude research aircraft, analyzing particles collected during multiple flights over a nine-year period. They found that the majority of cloud particles freeze, or nucleate（成核）, around two types of seeds: mineral dust and metallic aerosols.

The absence of certain particles in the clouds also proved interesting. While scientists have observed that substances like black carbon and fungal spores readily form cloud particles in the lab, the team detected barely a trace of these particles in the upper atmosphere.

"We think we're really looking at the seed, the nucleus of these ice crystals," says Dan Cziczo, an associate professor of atmospheric chemistry at MIT. "These results are going to allow us to better understand the climatic implications of these clouds in the future."

Cziczo and his colleagues have published their results this week in Science.