Cirrus Clouds Form Around Mineral Dust: How Humans Have an Impact

If you look up in the sky on a relatively clear day, you may see the thin wisps of vapor that mark the formation of cirrus clouds. These thin trailing clouds actually cover nearly one-third of the globe, forming in the upper layers of the troposphere more than 10 miles above the Earth's surface. Now, researchers have discovered a little more about these clouds and how they form.

Clouds form around "seeds"--small particles around which water vapor condenses. Since cirrus clouds form so high in the atmosphere--even higher than altitudes at which commercial aircraft fly--researchers needed to employ some special techniques in order to find information about them. Using a high-altitude research aircraft, the scientists collected particle samples that were gathered during multiple flights over a nine-year period.

Yet these flights had to be carefully scheduled. The researchers had to take weather forecasts into account and figure out exactly where and when cirrus clouds might be found.

"More often than not, the forecast is solid, and it's up to the pilot to hit a cloud," said Dan Cziczo, one of the researchers, in a news release. "If they find a good spot, they can call back on a satellite phone and tell us if they're inside a cloud, and how thick it is."

As the plane flew through a cloud, ice particles flowed through a specialized inlet into the nose of the plane. These particles then thawed, evaporating most of the surrounding ice. What was left over was a tiny kernel, or seed, which was then analyzed in real time by the onboard mass spectrometer.

The researchers found that the majority of cirrus cloud particles freeze, or nucleate, around two types of seeds: mineral dust and metallic aerosols. While mineral dust is largely regarded as a natural substance that originates from dry or barren regions on Earth, metallic aerosols are not. Yet even mineral dust is being affected by humans. Agricultural practices and industrial processes also release dust into the atmosphere.

The findings could allow researchers to create more accurate models for climate change. More specifically, increasing mineral dust could explain increasing cirrus cloud cover.

The findings are published in the journal Science.