The last new formation was cirrus intortus, added way back in 1951.
最后一个新的形态是卷云，早在 1951 年就已添加。
英语百科
Cirrus intortus cloud
Cirrus intortus is a variety of cirrus cloud. The name cirrus intortus is derived from Latin, meaning "twisted, wound". The variety of intortus clouds is specific to cirrus clouds, and they appear as interwound strands of cirrus clouds with a purely random pattern. The filaments are often curved in a very irregular pattern.