Atlantic Ship Tracks
March 17, 2003
An unusually high number of ship tracks were visible in the clouds off of the coasts of France and Spain in these images taken from the Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the Aqua satellite. The images were acquired on January 27, 2003. Ship tracks form when very small, airborne particles emitted in the exhaust of large ships attract water molecules, acting as “seeds” or cloud condensation nuclei. Continued accumulation of droplets on the cloud condensation nuclei forms the thin, streaky clouds pictured above. As ships moved about the world's seas, they sometimes leave a visible, though impermanent, record of where they've been. Instead of showing their previous location, like the contrail of a jet, ship tracks reflect the direction and speed of the wind.
The false-color cut-aways show two properties of clouds that influence the heat and energy balance of the atmosphere and, as a result, the climate. One is cloud optical thickness, which describes how much light is able to pass through a cloud. The other characteristic is cloud particle radius, which is the estimated size of the radius of the particles making up the clouds.
There are important difference between clouds formed from natural cloud condensation nuclei (like dust or sea salt) and those formed from particles in ship exhaust. First, the ship track clouds contain greater amounts of smaller water droplets than do surrounding natural clouds (shown in red). The optical thickness of the ship track clouds is different as well. A cloud’s optical thickness determines how much sunlight reaches the Earth’s surface and how much is reflected or absorbed by the clouds, factors that influence global temperatures. The size of cloud particles is important, too. In general, smaller particles produce brighter, more reflective clouds, which bounce sunlight back into space and cool the planet. See the Earth Science Picture of the Day for December 4, 2001.