dry out, expose to the sun
晞 centers on removing moisture by sunlight: drying something out or exposing it to the sun so it becomes dry.
晞 combines 日 (sun) with 希, which likely contributes the sound and may relate to the idea of something becoming sparse or thin as moisture evaporates.
The sun 日 on the left shines on the right side 希, like sunlight thinning out moisture until everything is dry.
For キ, imagine a key left in the sun: the hot key dries out completely, and key -> キ.