Roof of the mouth and Welkin

Roof of the mouth noun - The upper surface of the mouth that separates the oral and nasal cavities.

Welkin and roof of the mouth are semantically related. In some cases you can replace term "Welkin" with "Roof of the mouth", this terms are similar.

Welkin noun - The apparent surface of the imaginary sphere on which celestial bodies appear to be projected.

Roof of the mouth and welkin are semantically related. You can use "Roof of the mouth" instead a noun "Welkin".