万物简史 第374期:进入对流层(15)

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后来，他又增加了第4个名字，把一种会下雨的云叫做雨云（拉丁语的意思是“云”）。霍华德体系的妙处在于，这些基本成分可以自由组合，描述天空中飘过的每种形状、每种大小的云——一层积云、卷层云、积雨云等等。这个体系顿时取得成功，不仅仅在英国。歌德十分赞同这个体系，竟然写了4首诗献给霍华德。

Howard's system has been much added to over the years, so much so that the encyclopedic if little read International Cloud Atlas⁴ runs to two volumes, but interestingly virtually all the post-Howard cloud types—mammatus, pileus, nebulosis, spissatus, floccus, and mediocris are a sampling—have never caught on with anyone outside meteorology and not terribly much there, I'm told. Incidentally, the first, much thinner edition of that atlas, produced in 1896, divided clouds into ten basic types, of which the plumpest and most cushiony-looking was number nine, cumulonimbus. That seems to have been the source of the expression "to be on cloud nine."

在随后的岁月里，霍华德的体系又增加了许多内容，最后那部百科全书；性质的而又很少有人阅读的《国际云层图册》共有两卷。但是，有意思的是，霍华德去世以后确定的云的种类一一例如乳房状云、冠状云、雾状云、厚度云、絮状云和中度云，据说在气象学界外根本无人接受，在气象学界内接受的人也为数不多。顺便说一句，那本图册在1896年出的第一版，也是薄得多的一版，把云分成10个基本种类。其中，最丰满、最像垫子的卷叠积云排列第九。“九天云霄”这个表达方式似乎是由此而来的。

If you have ever been struck by how beautifully crisp and well defined the edges of cumulus clouds tend to be, while other clouds are more blurry⁵, the explanation is that in a cumulus cloud there is a pronounced boundary between the moist interior of the cloud and the dry air beyond it.

（积云往往干净利落，边缘分明，而别的云层模模糊糊。这是因为积云湿润的内部与外面干燥的空气有着明确的界线。

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