Anthropogenic Origins of Cirrus Clouds
"Cirrus" is Latin for a curling lock of hair so it is fitting that thin, wispy clouds that we often see in the atmosphere are called cirrus clouds. These clouds form when water vapor undergoes deposition at high altitudes and therefore are found at higher elevations and appear more delicate compared to the other types of clouds.
Cirrus clouds cover as much as one-third of the Earth and play an important role in global climate. Depending on altitude and the number and size of ice crystals, cirrus clouds can cool the planet by reflecting incoming solar radiation — or warm it by trapping outgoing heat.
Researchers at Oregon State University have been studying the origin of these clouds because of their unique formation and have found that these thin trails of ice crystals are formed primarily on dust particles and some unusual combinations of metal particles — both of which may be influenced by human activities.
"Cirrus clouds are complicated but the important message is that dust and certain metals provide the seeds for a majority of the ice crystals that form the clouds," said Cynthia Twohy, an Oregon State University atmospheric scientist and co-author of the study. "Other particle types — including bacteria and soot from human-produced combustion or natural sources — don't seem to contribute much to the nuclei of cirrus crystals.
"These biological particles may be important in the formation of lower altitude clouds," added Twohy. "But they were surprisingly absent from the particles we sampled from cirrus clouds."
Researchers found that about 60 percent of the cloud particles they analyzed could be traced to mineral dust blown into the atmosphere, or to metallic aerosols.
"Mineral dust can occur naturally," Twohy said, "or it can be influenced by human activities. Certainly the major deserts like the Sahara and Gobi are enormous sources of mineral dust. But agriculture, over-grazing and climate and land-use changes can also contribute."
"At lower altitudes, clouds are known to be influenced by pollution — especially near cities," Twohy said. "They have more droplets, they reflect more light and they rain less. The impacts of cirrus clouds on climate are much more complex. But this gives us a starting point because we now have a better understanding of the particle types and mechanisms that lead to their formation."
Results of the study were published this week in the journal Science.
Read more at Oregon State University.
Cirrus clouds image via Shutterstock.