Earthshots: Satellite images of environmental change – Selkirk Island, Chile
Earthshots introduces remote sensing by showing examples of how environmental changes look from space.
These images show cloud vortices that formed with unusual clarity over Selkirk Island, just over 500 miles off the coast of Chile. These whorls formed as the tall, steep island interrupted the northward-sweeping clouds. We ordinarily show as few clouds as possible in Earthshots because they block our view of the land, but here we want to show off their scientific and especially their aesthetic value.
Selkirk Island is the tip of a volcanic peak, rising sharply from the sea. Though covering only 33 square miles, the island rises over a mile into the sky.
In September 1999, as engineers at EROS reviewed Landsat sensor output, they noticed unusual clouds over Selkirk Island. They referred this image to EROS scientists, who identified the series of swirls as a “Karman vortex street,” a fluid flow pattern rarely seen so clearly outside of a laboratory. The American Meteorological Society featured the Selkirk image on the cover of their monthly bulletin.
How did these Karman vortices develop? On that day, the wind was carrying northward a layer of stratocumulus clouds (flat-bottomed puffballs). The mile-high island caused this cloud layer to slow about the island, while remaining fast farther out on either side. So on each “wing,” left and right, the air started rotating toward the inside—clockwise on the left, counter-clockwise on the right. The rotational momentum made each side swirl in on itself. The whorl-cores were clear because the swirling pulled dry, clear air (from above or below) into the wet layer, a bit like the funnel formed when you stir up a pitcher of orange juice. These clear, spinning pockets trailed off down the “street” from the island like soap bubbles from a toy wand—drifting downwind, weakening, filling with clouds, and breaking up.
Additional publication details
|Title||Earthshots: Satellite images of environmental change – Selkirk Island, Chile|
|Publisher||U.S. Geological Survey|
|Contributing office(s)||Earth Resources Observation and Science (EROS) Center|
|Other Geospatial||Selkirk Island|
|Online Only (Y/N)||N|
|Additional Online Files (Y/N)||N|