It's also unclear how cirrus clouds might affect the climate. Depending on their location in the atmosphere, they can either help cool the Earth or warm it up. Unlike liquid water clouds, which generally cool the Earth by reflecting sunlight, ice clouds can help warm it up by absorbing reflected heat, Toon said.
Cirrus clouds are unique in that they are made up of ice crystals, whereas most other clouds contain condensed droplets of liquid water. When most people think of clouds, they picture these liquid water clouds, which exist closer to the Earth's surface and are responsible for rainstorms and other weather, Cziczo said.
Like all clouds, water clouds need particles upon which to condense. Recent research has shown that these clouds can form on airborne microbes, sulfates (chemicals emitted by volcanoes and human activity) and other organic materials, Cziczo said. As this study shows, however, cirrus clouds need very specific particles upon which to nucleate and freeze.
Analyzing chemicals within high-altitude ice crystals is no easy task. To do so, Cziczo and his colleagues partnered with NASA to use two of its research airplanes. Up in the air, they used a device called a counterflow virtual impactor. This device acts like a hair dryer, Cziczo said.