And there was evening
In the process of creating the earth all the parts of the day were brought into existence at once. Though things might seem flat, the earth, as created, was demonstrably a sphere. The light of day disappears gradually only when the globe turns away from the light source.
The two main divisions of the the day, in a general sense, were the daylight and the dark night. These two were separated, but there remains a diminishing light after the sunset. This period is called evening, or עֶ֥רֶב (`ereb).This is from the verb. עָרַב (`arab), to braid or intermix. It is an apt word only partly conveyed by the English evening.
As our world rotated away from the now localized light of God's presence, the creation could mix cold waters with the energetic light-infused waters, moderating the temperature of the whole earth.