From: Andy Soos, ENN
Published August 26, 2011 11:35 AM

Cloud Formation

Clouds are amazing to watch and intricate in their formation and interactions with the atmosphere. Research from CERN involving University of Leeds scientists provides news insights into cloud formation in the atmosphere. In a paper published in the journal Nature today, the CLOUD experiment - designed to study the effect of cosmic rays on the formation of atmospheric aerosols under controlled laboratory conditions - reports its first results. Aerosols are tiny particles suspended in the atmosphere which are thought to be responsible for a large fraction of the seeds that form cloud droplets. Understanding the process of aerosol formation is therefore important for understanding the climate.


A cloud is a visible mass of water droplets or frozen ice crystals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in the Earth's atmosphere are studied in the nephology or cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Generally, precipitation will fall to the surface; an exception is virga which evaporates before reaching the surface. Clouds can show convective development like cumulus, or in the form of layered sheets such as stratus, or appear in thin fibrous wisps as in the case of cirrus. Prefixes are used in connection with clouds: strato for low cumulus-category clouds that show some stratiform characteristics, nimbo for low to middle stratiform clouds that can produce moderate to heavy precipitation, alto for middle clouds, and cirro for high clouds. Whether or not a cloud is low, middle or high, level depends on how far above the ground its base forms.

The CLOUD results show that trace vapors assumed until now to account for aerosol formation in the lower atmosphere can explain only a tiny fraction of the observed atmospheric aerosol production. The results also show that ionisation from cosmic rays significantly enhances aerosol formation.

Co-author Professor Ken Carslaw from the University of Leeds, said: "Twenty years of research has told us that the formation of atmospheric particles is important for Earth's climate. Now the CLOUD experiment has given us a unique insight into how the particles form and will completely change our understanding of what's happening in the atmosphere."

Atmospheric aerosols play an important role in the climate. Aerosols reflect sunlight and produce cloud droplets. Additional aerosols would therefore brighten clouds and extend their lifetime.

Trace sulfuric acid and ammonia vapors are thought to be important, and are used in all atmospheric models, but the mechanism and rate by which they form clusters together with water molecules have remained poorly understood until now.

The CLOUD results show that a few miles up in the atmosphere sulfuric acid and water vapor can rapidly form clusters, and that cosmic rays enhance the formation rate by up to ten-fold or more.

However, in the lowest layer of the atmosphere, within about a mile above the Earth's surface, the CLOUD results show that additional vapors such as ammonia are required.

Crucially, however, the CLOUD results show that sulfuric acid, water and ammonia alone - even with the enhancement of cosmic rays - are not sufficient to explain atmospheric observations of aerosol formation. Additional vapors must therefore be involved, and finding out their identity will be the next step for CLOUD.

"It was a big surprise to find that aerosol formation in the lower atmosphere isn't due to sulfuric acid, water and ammonia alone," said Kirkby. "Now it's vitally important to discover which additional vapors are involved, whether they are largely natural or of human origin, and how they influence clouds. This will be our next job."

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