1. It's cold as hell outside.
2. It's as cold as hell outside.
I've often seen the two way of the usage of "as hell" like in 1 and 2, but personally I'm not inclined to think that 1 is a grammatical sentence, because "as" isn't in front of "cold".
What I wonder about is that whether 1 can be considered grammatical even though "as" isn't in front of "cold". Maybe it can be? because "as" is often informally omitted like in 1?
To put it simply, I'm not sure whether both 1 and 2 are grammatical.
fire1 I'm not sure whether both 1 and 2 are grammatical. You can be sure they are both grammatical. as is used in common similes A simile is an expression where one thing is compared to another to give emphasis.
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fire1I'm not sure whether both 1 and 2 are grammatical.
You can be sure they are both grammatical.
See
https://www.eltbase.com/mats/20180205065622grammar-notes-as-as.pdf
As...as is used in common