clear, high
杲 centers on brightness and elevation: the sun high above a tree, making things clear and visible. The meanings 'clear' and 'high' both stem from this image of the sun at its peak.
杲 combines 日 (sun) above 木 (tree), depicting the sun high over a tree to express brightness and height. The exact historical development is uncertain, but the component arrangement is consistent with this interpretation.
Picture the sun 日 rising above a tree 木: the sun is high and the sky is clear. 杲 is the bright, high sun.
For コウ, imagine a bright, high sun shining over a tree, and think of the word 'core' (コウ) of the sky—the sun at its peak.