January 21, 2008
They look like airplane trails, but the streaky clouds shown in this photo form around the exhaust left behind by ships sailing along the northwest coast of North America. All clouds form when water molecules combine with tiny particles in the atmosphere. These particles can be natural substances such as dust or sea salt, but they can also be aerosols released by human activities. Because only the water surface affects it, the air over the ocean tends to be uniform, often forming flat clouds that cover a wide area. When a ship introduces new particles into the air by emitting a puff of exhaust, water molecules can easily stick to these particles, forming a trail called a ship track behind the ship.
Scientists are interested in ship tracks because they want to understand how human emissions affect clouds and ultimately the Earth's climate. Emissions from factories, power plants and cars are mostly released onto land where the air is turbulent. It's difficult to track emissions in the constantly churning air, and even harder to determine how they affect cloud formation. On the other hand, ship emissions end up in the relatively calm air over the ocean, and the clouds formed by ship exhaust are significantly different from other clouds.
Ship tracks show that clouds that form around man-made aerosols are brighter than other clouds. Artificial aerosols are smaller than natural particles, so the clouds that form around artificial aerosols are made up of smaller cloud droplets . A cloud composed of many small droplets reflects more light than a cloud composed of several larger droplets because the surface of each droplet reflects light. Brighter clouds created by man-made aerosols reflect more sunlight back into space, reducing the amount of light that reaches the Earth's surface. The interaction of man-made aerosols with clouds is cooling the Earth, offsetting the effects of global warming, although scientists aren't sure how much. More accurate predictions of future climate warming depend on understanding how much cooling brighter clouds provide.
Ship tracks also suggest that man-made aerosols can affect rainfall in some environments. Smaller water droplets brighten clouds and may prevent rainfall because the water does not gather into large enough clumps to condense into rainfall. Therefore, cloud formation around man-made aerosols may reduce rainfall in some environments. In other environments, such as the hot and humid climate of the southeastern United States in the summer, aerosols appear to increase rainfall because they intensify thunderstorms.
Moderate Resolution Imaging Spectroradiometer (MODIS) in NASA aqua green On January 21, 2008, the satellite captured this photo of ship tracks in the Pacific Ocean.
