Ship tracks

  Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles (aerosols) from exhaust to form a cloud seed. More and more water accumulates on the seed until a visible cloud is formed. In the case of ship tracks, the cloud seeds are stretched over a long narrow path where the wind has blown the ship's exhaust, so the resulting clouds resemble long strings over the ocean.

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Ship tracks study

Scientists who study how human-produced aerosols affect cloud formation observe ship tracks because in most urban areas, they are unable to discern exactly how pollutants contribute to forming clouds because the atmosphere over the land is too tumultuous. In contrast, ships (not significant sources of pollution themselves) release their exhaust into the relatively clean and still marine air, where the scientists have an easier time of measuring the effects of fossil fuel emissions on cloud formation.

In general, the air above the oceans suffers from less turbulence and convection than the air above land. The lower atmosphere is especially calm over the eastern Pacific in the summertime due to a layer of hot air that settles in 500 to 700 meters above that region of the ocean. This effect creates a temperature inversion, placing a cap on the cooler air below, trapping pollutants and water vapor. While the inversion is responsible for the smog that reduces air quality in Los Angeles, it also allows for the formation of long lasting ship tracks. The particles billowing from ships' smokestacks enter the air above the eastern Pacific and create long, thin clouds that remain there for days.

Findings

What they have found is that the sulfur dioxide released from ships' smokestacks could be forming sulfate aerosol particles in the atmosphere, which cause the clouds to be more reflective, carry more water and possibly stop precipitating. This is proof that humans have been creating and modifying clouds for generations through the combustion of fossil fuels.

Although ship tracks can sometimes be visible, researchers usually scan the near-infrared light coming off the clouds. At this wavelength many ship tracks appear as bright lines that can be distinguished from the surrounding, uncontaminated clouds. On average, polluted clouds reflect more sunlight than their unaffected counterparts.

When compared to normal clouds, the number of water droplets per cubic centimeter in ship tracks are more than doubled, the radius of the drops is decreased by roughly six percent and the amount of water per cubic meter is increased twofold. In other words, this excessive cloud seeding from the ship causes the clouds to retain more water. Normally, rain forms when cloud drops coagulate and reach a size where gravity can pull them to the ground. Yet, in ship tracks, the cloud seeding made the drops so small that they could no longer easily merge together to reach the size needed to escape. Since no drizzle came out of the seeded clouds, the liquid water just kept building in the cloud. This makes the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.

Implications

The study of ship tracks may go a long way towards explaining some of the climatic mysteries in the world, such as why global warming is affecting the Southern Hemisphere much more quickly than the Northern Hemisphere. The dominant hypothesis in the scientific community today is that aerosol forming pollution is greater in the Northern Hemisphere than in the Southern Hemisphere and this contributes a mitigating influence on global warming expected from the greenhouse effect.^{[citation needed]}

The effects of the sulfur dioxide from industry might be countering the greenhouse effect created by carbon dioxide, but only in the Northern Hemisphere. When fossil fuels are burned, both carbon dioxide and sulfur dioxide are released. As studying ship tracks has shown, sulfate particles produced from sulfur dioxide creates brighter clouds, which may cool the atmosphere. Any light that is reflected cannot reach the ground and heat the surface of the Earth. This means there is less heat for carbon dioxide and other greenhouse gases to trap in the atmosphere.

Another implication is for navies who attempt to hide their ships underneath cloud cover, as this apparently does not work very well.

See also

• Contrail
• Global dimming

References

• Ship Tracks over the Atlantic. NASA Earth Observatory. Retrieved on 2006-05-11.
• Weier, John (September 30, 1999). Every Cloud Has a Filthy Lining. NASA Earth Observatory. Retrieved on 2006-05-11.

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