darkness, dimly
杳 centers on deep, obscure darkness: a state where light is faint or absent, making things dim and hard to perceive.
杳 combines 木 (tree) above 日 (sun), suggesting the sun sinking below the tree line into darkness. The exact historical development is uncertain, but the component arrangement visually conveys dimness.
Picture the sun 日 setting beneath a tree 木, leaving the world in deep, dim darkness.
For ヨウ, imagine the darkness slowly swallowing the last light: 'yo…' the light fades, and the dimness takes over. The drawn-out 'yo' sound cues ヨウ.