Are the following expressions natural?
1. It is getting dark.
2. It is growing dark.
3. It is becoming dark.
4. It is turning dark.

Yes. All are certainly grammatical. #3 may occur less often because in
less-formal language 'getting' is usually used instead of 'becoming'. #1
is very natural and less formal; 2 & 4 strike me as slightly literary
(esp. 2), and 3 is not literary, but does strike me as more formal than 1.

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Johanna Rubba   Assistant Professor, Linguistics
English Department, California Polytechnic State University
One Grand Avenue  • San Luis Obispo, CA 93407
Tel. (805)-756-2184  •  Fax: (805)-756-6374 • Dept. Phone.  756-259
• E-mail: [log in to unmask] •  Home page: http://www.calpoly.edu/~jrubba
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"Understanding is a lot like sex; it's got a practical purpose,
but that's not why people do it normally"  -            Frank  Oppenheimer
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