皓 is a bound morpheme - it doesn't stand alone. Common compounds: 皓月 (bright moon), 皓齿 (white teeth), 皓首 (white-haired, i.e. old age). Famous in Fan Zhongyan's 岳阳楼记: 皓月千里 (the bright moon shining a thousand li).
memory
皓 = 白 (white) + 告 (gào, phonetic). Picture announcing (告) something white (白) - the moon, teeth, or aged hair. The 白 component is your meaning anchor.
白 is the indexing radical and carries the brightness sense directly - 皓 means dazzling white, like moonlight, snow, or white teeth. Same radical heads 皑, 皎, 皙 - all words for shades of whiteness.
告 gives the sound, drifting from gào to hào - the g-/h- alternation is well attested in Old Chinese. No semantic link; same phonetic appears in 浩 (vast), 皓 (bright), 鹄 (swan).
