Is it correct to say 明天[tian qi]比今天冷吗? Tomorrow's weather will be colder than today?
There is a missing word: 明天天气比今天-的-冷吗
I think not. You don't have to use the word 'weather' here at all.
Couldn't we say, "明天比今天很冷吗"?
One textbook I used in the past says you do not use 很 with 比.
"明天會比今天冷吗？" should be the correct answer. There is currently no verb in the solution sentence.
The sentence works without 'bi', but may be confusing since it omits the future time reference that 'hui' would provide.