Allusion is a historical metaphor, of which the original scene or scenario is abstracted into a phrase or a few words and the mapping relationship with what is described is in a connotative position.
典故是一种历史化隐喻，其原义的具体情节常被抽象化，并且与所描述的现时事件发生喻义上的冲突或错置，映现关系也处于隐含状态。
The hole isn't literal; no part of the stratosphere is empty of ozone. But scientists use the hole metaphor to describe the area where ozone drops below a historical concentration of 220 Dobson Units.
这里所谓的“空洞”不是字面上含义，并不存在任何一处平流层完全没有臭氧，科学家们用空洞这一说法来描述这些臭氧浓度低于历史读数220多布森单位的区域。
Scientists use "hole" as a metaphor for the area in which ozone concentrations drop below the historical threshold of 220 Dobson Units.
科学家们用“空洞”一词表示臭氧浓度低于220个多布森单位（多布森单位是用来度量大气中臭氧柱尺度的单位。
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