下半天

• 一(horizontal line; boundary)RADICAL
• 卜(stroke descending downward)ASSOCIATIVE

A vertical stroke pointing downward across a boundary line marks 'below'.

• 丷(two diagonal strokes; division in half)ASSOCIATIVE
• 牛(cow; cattle)ASSOCIATIVE

Using a knife to split a cow in half equally shows the meaning of 半 (half).

• 大(adult; large (pictographic))RADICAL
• 一(one; top)ASSOCIATIVE

The sky is the vast, boundless expanse that lies above human beings.