Orphan anvils are so called because they are solitary anvils from thunderstorms that have dissipated. Usually, a thunderstorm dissipates at its lower regions first, where the cloud droplets have not been frozen in ice crystals. Ice crystals take much longer to evaporate. Orphan anvils often show the upper parts of the storm's former downdraft well, as virga (a snow fallstreak that doesn't reach ground). The technical name for orphan anvils would be cirrus spissatus, although these orphan anvils are usually much deeper than just at the cirrus level.
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