阴 is the hill 阝 with the moon 月: the northern slope where sun never lands. Its twin 阳 carries the sun. Every Chinese hill has an 阴 face and an 阳 face — and from those two slopes came the cosmology of yin and yang.
阴天 is an overcast day. 阴阳 yin-yang divides everything: moon/sun, cold/hot, quiet/loud, the two interlocked commas of the taiji symbol. Place names remember the geography: 江阴 sits on a river's shaded south bank.
Moon 月 side of the hill 阝 — SHADED, CLOUDY, YIN.
阴 also guards the underworld: 阴间 is the realm of the dead, patrolled by 阴差 — ghost constables. The weather report and the afterlife share a character.