What does "a cloud pass across her gaze" mean?

Question

From the novel The Spirit of the Place by Samuel Shem:

Selma stared at him. He saw a cloud pass across her gaze. She sighed. "Orville-doll, there's nothing else but this."

What does "a cloud pass across her gaze" mean?

Answer 1

This phrase is a less-common variant of a common metaphor which uses cloudy weather to represent negative emotion visible in a person's face. Here are some dictionary sources to demonstrate some other variants of this metaphor:

If a person's face clouds over, they suddenly look unhappy or worried:
At the mention of her dead husband, her face clouded over.

_{-- Cambridge Dictionary, cloud over, meaning 2}

Used to refer to a state or cause of gloom, suspicion, trouble, or worry.
‘the only cloud on the immediate horizon is raising a mortgage’

A frowning or depressed look.
‘a cloud passed over Jessica's face’

_{-- Lexico, cloud, meaning 2}

If your eyes or face cloud or if sadness or anger clouds them, your eyes or your face suddenly show sadness or anger.
[literary]
Trish's face clouded with disappointment.
As he looked at Katherine, great sorrow clouded his eyes.

_{-- Collins Dictionary, cloud, meaning 6}