Off the west coast of South America, an intricate network of clouds presented a spectacular view to the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite when it flew overhead on September 30, 2005.
Here, the Pacific Ocean is covered by three closely related cloud types: open-cell, closed-cell, and actinoform clouds. Open cell cloud formations have open space in their centers fringed by clouds. Closed-cell cloud formations are the opposite: a cloud “block”in the center separated from adjoining blocks by a perimeter of clear air. Actinoform clouds have a ray or leaf-like shape.
In this image, open-cell clouds appear in the north, while a distinct leaf-like actinoform cloud appears to the southwest of the open-cell clouds. These lace-like clouds were first observed by the Television Infrared Observation Satellite Program (TIROS) operated by NOAA in the early 1960s. Researchers used to think that actinoform (also spelled actiniform) clouds were transitional forms between open- and closed-cell formations, but actinoform clouds appear to be their own brand of cloud formation.
These open- and closed-cell cloud formations result from convective cells, which are loops of rising and sinking air. As a general, atmospheric rule of thumb, clouds form in places where the air is rising, and therefore cooling and causing more water vapor to condense. The reverse of that is true as well, clear areas suggest where air is sinking, and therefore warming, making cloud formation less likely.