Droughts are recurrent phenomena that present a large variety of space and time patterns making rather difficult the assessment of their rarity and the comparison between events.

中文翻译：

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多时间干旱稀有度曲线 - 法国气象干旱严重程度的年度分类


干旱是反复出现的现象，呈现出多种不同的空间和时间模式，这使得评估其稀有性和事件之间的比较相当困难。