This article is from the Storms FAQ, by Chris Landsea landsea@aoml.noaa.gov with numerous contributions by others.
"CDO" is an acronym that stands for "central dense overcast". This is the
cirrus cloud shield that results from the thunderstorms in the eyewall of a
tropical cyclone and its rainbands. Before the tropical cyclone reaches
hurricane strength (64 kt or 33 m/s), typically the CDO is uniformly showing
the cold cloud tops of the cirrus with no eye apparent. Once the storm
reaches the hurricane strength threshold, usually an eye can be seen in
either the infrared or visible channels of the satellites. Tropical cyclones
that have nearly circular CDO's are indicative of favorable, low vertical
shear environments.
 
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