A heavy cloud cover shrouded the South Sandwich Islands in late March, 2013, hiding a smoldering volcano from view. Those same clouds, however, served as a canvas to illustrate the eruptive activity, as well as the strong winds blowing across the region. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASAís Terra satellite captured this true-color image of the volcanic plume from the Mount Michael volcano on March 28.
The islands captured in this picture are all volcanic in origin, with steep, mountainous terrain. The tall peaks form a blunt barrier against the roaring winds that blow across the open ocean. When wind slams into the immobile terrain, it is forced up and around, creating turbulent air flow on the lee side. The wind-formed cloud patterns created by the turbulent air behind each island are known as ship-wave-shaped wave clouds, because they look similar to waves left in the wake of a ship traveling through water.
Although the turbulent wind creates patterns behind each of the islands, the plume from Mount Michaels trails behind Saunders Island, the second island from the top. It appears similar to a bright white ribbon blowing far eastward across the southern Atlantic Ocean.
Large, highly energetic eruptions can shoot ash and gas far into the atmosphere, creating huge, dark ash-filled plumes. Mount Michaelís eruptions tend to be steady, simmering and small, emitting primarily gases and steam, and such gas-filled plumes appear bright white. Although volcanic gas itself canít be seen, the gases in volcanic emissions can act as seeds for cloud formation. Because the aerosols (sulfates) in volcanic emissions are very tiny, the cloud droplets formed are also very tiny Ė substantially smaller than those found clinging to cloud-forming seeds of natural aerosols, such as sea salt or dust. Because the droplets are small, they can pack densely together, and therefore reflect more light than normal clouds. That is why, from space, the volcanic plume appears brighter white than the surrounding clouds.