NASA MODIS Image of the Day: September 22, 2011 – Cloud vortices off Heard Island, south Indian Ocean
Heard Island is located in a windy and remote part of the southern Indian Ocean, near the meeting point of more temperate sub-Antarctic surface waters and colder Antarctic surface waters.
It is far from any large land mass, lying about 1,550 kilometers north of Antarctica and over 4,000 kilometers south-west of Australia.
Surrounded by oceans, the relative humidity near the island is very high – usually around 80%. This high humidity along with the prevailing strong westerly winds known as the “furious fifties” which blow persistently across Heard Island, create excellent conditions for persistent cloud cover. Add a rugged and steep topography and the stage is set for the creation of spectacular images in the clouds. On September 19, 2011 the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite flew over this region, and captured a stunning true-color image of cloud vortices off of Heard Island. As the driving winds blow against the looming bulk of the Big Ben volcano, the major topographical feature of the island which rises to a peak height of 2,745 meters, the winds are blocked as they slam into the massive obstacle. The wind then changes speed as it is forced past the island, and the airflow spins into swirling loops of moving air on the leeward side. This particular pattern is also known as a von Karman vortex street, which is a linear chain of swirling eddies that form nearly anywhere that the flow of fluid or air is abruptly disturbed by an object. The Big Ben Volcano is currently considered to be active, with the last eruption occurring in 2008. Occasionally, satellite images will capture what appears to be a volcanic plume coming from Mawson Peak, which sits within a breached caldera on the southwestern side of the volcano. In this image, a bright white plume-like streak can be seen over the nearly cloud-free zone to the northwest side of the island. Although it looks somewhat like a volcanic plume, it is actually a long, thin, “banner cloud”, another type of cloud that often forms when air flows around, rather than over, a steep mountain peak, and does not indicate volcanic activity.
