"For the Earth’s atmosphere to form clouds at all, you need
particles for the water vapor to condense on," said Michael King. He is an
atmospheric scientist at NASA’s Goddard Space Flight Center who has studied
cloud formation for more than ten years. He explained that when water evaporates
into the atmosphere, it does not move together on its own to form water
droplets, but spreads evenly throughout the atmosphere. Airborne particles
(aerosols) that dissolve easily in water, such as ammonium sulfate and sea salt,
break up this otherwise even distribution and give the water molecules something
to cling to. The particles provide nuclei for cloud droplets to condense around,
and together these droplets form clouds. Were it not for particles in our
atmosphere, the sky would always be clear and the air around us, thick and humid
(King et al. 1995). |
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King said, "A greater number of aerosol particles in a cloud will lead
to the production of more cloud droplets, albeit smaller in size, than would
otherwise be the case. This in turn increases the brightness of the cloud."
Additional particles give the water vapor more nuclei to cling to, so more
smaller drops form in the cloud. These smaller drops in turn make the cloud
more reflective to sunlight. The same phenomenon can be seen when ice cubes are
crushed. As the ice is broken up, the once smooth surface is shattered into many
tiny surfaces at varying angles to one another. These additional surfaces
reflect incoming light in all directions and cause the crushed ice to appear
white and opaque. Water droplets do not contain all these ridged surfaces, but
their fragmentation has a similar effect on sunlight.
A majority of cloud-creating aerosols arise from natural sources, ranging
from volcanoes to microscopic ocean plants. Yet, scientists speculate that an
ever growing number originate from our need for electricity and transportation.
Each time we burn fossil fuels, sulfur dioxide, a gas that leads to the
formation of sulfate aerosols, is released. The particles often rise into the
atmosphere and create more and brighter clouds. Though no one is certain what
effect this extra cloud cover may have on the environment, many scientists
believe it may be cooling the atmosphere. "By modifying clouds we make them
brighter," King said. "This has a cooling effect on the climate,
because the clouds are reflecting more solar radiation than they normally do
back into space." Any light that is reflected cannot heat the ground or the
atmosphere.
Proving this hypothesis is much more difficult than measuring the brightness
of the clouds over a smokestack. Above land, where most industry is located, the
wind currents and convection are too tumultuous to measure cloud formation from
individual pollution sources. The clouds break up and change so often that
scientists have trouble monitoring the aerosols, let alone determining the
precise amount of fossil fuel needed to create clouds. What the researchers
needed was a pollution source that spilled contaminants continuously into a calm
and relatively pristine air mass (King et al. 1995).
Searching for Ship Tracks
Every Cloud Has a Filthy Lining |
>Whole ice cubes transmit more light than they scatter, (clear ice, top)
compared to crushed ice which scatters light very well (white opaque ice, above.) This is analagous to
the bright white clouds formed with high amounts of aerosols, which are composed of many
very small water droplets. (Photographs by Robert Simmon, NASA Earth Observatory)
For more information, see What Are Aerosols? |