How Traditional Ice Cubes Freeze and Why They Are Cloudy

In a traditional ice cube tray, which is not insulated on any side, cool air hits all sides of the tray. Ice forms on the top, bottom, and sides, and freezes from the outside toward the center. It is the center part of an ice cube where it is cloudy and cracked (the cracking through pressure because ice expands as it freezes), while the outsides are typically clear.

We say that ice wants to freeze into a perfect crystal lattice (though want isn't the proper term) and trapped impurities and air prevent it from doing so. When water freezes, it pushes any trapped air and impurities away from the first part to freeze. In a traditional ice cube, this is towards the center of the cube. Using directional freezing, it pushes the air/impurities/increase in pressure towards the bottom of insulated cooler (or the last part of any controlled-direction freezing container).