Clouds are composed of billions of water droplets or ice particles. When the air is saturated with moisture, water vapor changes phases into liquid water (condensation). The liquid water collects on particles in the atmosphere known as cloud condensation nuclei (CCN). Some aerosols, such as sea salt, sulfates, and nitrates are hygroscopic, which means they can accelerate water condensation so that the air does not have to be fully saturated with moisture.

There is a limited amount of moisture in the air to collect on CCN. Clouds with fewer CCN have larger water droplets because the moisture does not have to be spread out. The introduction of human produced aerosols into the atmosphere increases the number of particles available to become CCN. As a result the clouds have more and smaller water droplets. This increases the cloud’s reflectivity and makes them last longer.

Clouds with low aerosol concentration (left) and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface. Clouds with high aerosol concentration (right) reflect up to 90% of light back to space.
courtesy of NASA Earth Observatory

For clouds to produce rain, water droplets have to reach a sufficiently large size. When two small droplets collide they can combine to form one large drop. This process will continue until the drops are big enough to produce rain. However, the likelihood of collision decreases for smaller drops, so in a cloud with many small droplets this process can take longer than the usual amount of time. When the rain does fall, the cloud may have been carried farther than average from its point of origin, possibly affecting the natural amount of rainfall an area should receive.

URLs: Clouds

http://www.etl.noaa.gov/et2/aca/

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/dvlp/upw.rxml

Cloud Types
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/cldtyp/home.rxml

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