Searching for Ship Tracks

"Our interest in ship tracks came about quite by accident," said James Coakley, an atmospheric scientist at Oregon State University. He came up with the idea to study ship tracks back in 1985 while visiting Scripps Institution of Oceanography to learn about the newly-burgeoning satellite imaging technology. He said the first image shown to him was of the eastern Pacific off the coast of California. "I saw these bright streaks running through the images," he said. "Immediately I knew they were ship tracks." Coakley also saw that the ship tracks and the atmospheric conditions off of California would be ideal for studying the effects of man-made aerosols.

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, Coakley explained. 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 bellowing from ships’ smokestacks enter the air above the eastern Pacific and create long, thin clouds that remain there for days.

Coakley and his colleagues began a general survey of the ship tracks with satellite imagery. They took a number of images of the eastern Pacific using the Advanced Very High Resolution Radiometer (AVHRR). Built and managed by the National Oceanic and Atmospheric Administration (NOAA), multiple copies of this satellite instrument circle the Earth in near-polar orbits. Together, they scan the entire surface of the planet. The instruments do not simply take a picture of an area of the Earth, but use light sensors to detect specific colors (wavelengths) of light and thermal radiation coming off the Earth. These readings are beamed back to Earth in the form of data, which the scientists can manipulate to form images.

This photograph from the Space Shuttle shows marine layer clouds near Baja California. These clouds often stretch for hundreds of thousands of square miles in stable air just above the surface of the ocean, and provide a convenient laboratory for the study of the interaction of clouds and aerosols. (Photograph courtesy NASA Johnson Space Center)

When searching for ship tracks, Coakley’s team looked at the near-infrared light (light which has a wavelength longer than that of red light in the visible color spectrum) coming off the clouds. At this wavelength many ship tracks appear as bright lines that can be distinguished from the surrounding, uncontaminated clouds. Once the scientists located polluted clouds, they made additional measurements of reflected light in the visible range. "On average, we found the polluted clouds reflect more sunlight than their unaffected counterparts," said Coakley.

Seeing the Clouds Close-Up
At the Core of Cloud Formation

This pair of ship tracks, barely discernible in visible light (top,) stand out clearly in the near infrared (above.) (Images by Robert Simmon, based on NOAA Advanced Very High Resolution Radiometer (AVHRR) data)