NASA MODIS Image of the Day: May 16, 2011 – Cloud vortices off Isla Socorro, North Pacific
The cloud cover of the North Pacific Ocean was decorated with lacy swirls and bold stripes on May 11, 2011, when the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite flew over the region and captured this true-color image.
Cloud patterns are beautiful to behold, but formations within cloud cover have more in common with cryptographs than pure art.
Each swirl, stripe and even the color of a cloud reveals a story for those who know how to read such patterns. The most striking pattern in this image is the string of repetitive vortices that trail southwards from the green Isla Socorro. This formation, known as a Karman vortex street, reveals turbulent wind flow. From the direction that the pattern lies across the sky, it is clear that a strong wind blows southward. As the wind slams against abrupt rise of the 1,050-meter-tall summit of the volcanic island, it lifts up and is blown around either side of the obstruction, causing a turbulent flow. This is first revealed where the cloud cover ends in a sharp arc around the front of the island. Then the wind behind the island flows in repetitive swirls, much like eddies formed on a strong inflowing tide that runs against a dock’s piling, and the swirling pattern of the clouds reveal the swirling path of the wind. To the northeast of Isla Socorro, the clouds form a sharp stripe across the sky. Here the strong southward-flowing wind encounters the tiny Isla San Benedicto, whose tallest peak, at 332 meters, is tall enough to obstruct the flow, but not big enough to cause the swirling turbulence formed behind the bigger mountain. Instead, the wind ripples around the island in a cloud-free V-shape, then bright white clouds reform along a southward flow directly behind the island. Most of the cloud cover in the image appears dull white which tends, in some area, to light gray. This coloration is revealing, as well. Clouds are formed when many tiny droplets of water or ice condense around a nucleus. These tiny droplets are packed tightly together, and reflect all visible wavelengths of sunlight. Because of the wavelengths reflected, the color appears white. Because of the tiny size of the droplets, the reflectivity is high and the white appears very bright. When tiny droplets in clouds begin to merge with others, a process called accumulation, the space between each droplet becomes larger, allowing the cloud to absorb more light. Because of the increased light absorption, the reflectivity is less, so a cloud made up of large droplets can appear dull white. As this accumulation continues, the diminished reflectivity can result in clouds that appear gray.
