dag (n.)

"露水，指在夜间通过冷凝从大气中沉积下来的水蒸气"，中古英语 deaw, deu，源自古英语 deaw，源自原始日耳曼语 *dawwaz（源头还包括古撒克逊语 dau，古弗里西亚语 daw，中古荷兰语 dau，古高地德语 tau，德语 Tau，古诺尔斯语 dögg "dew"），或许源自 PIE 词根 *dheu- "流动"（源头还包括梵语 dhavate "流动，奔涌"）。

引申意义上用于指清新的事物（古英语晚期），或者暗示清晨和青春的新鲜（1530年代）。作为动词，指"用露水或像露水一样弄湿"，古英语 deawian。

The formation of dew is explained by the loss of heat by bodies on the earth's surface through radiation at night, by which means they and the air immediately about them are cooled below the dew-point ....Dew is thus deposited chiefly on bodies which are good radiators and poor conductors of heat, like grass; hence also it appears chiefly on calm and clear nights--that is, when the conditions are most favorable for radiation. It never appears on nights both cloudy and windy. In winter dew becomes hoar frost. [Century Dictionary]

露水的形成是通过地面物体在夜间通过辐射失去热量，从而使它们和周围的空气冷却到露点以下来实现的。因此，露水主要沉积在良好的辐射体和热导率较差的物体上，如草地； 因此，它主要出现在风平浪静、晴朗的夜晚——也就是辐射条件最有利的时候。它从不出现在既多云又有风的夜晚。在冬季，露水变成霜。[世纪词典]