We use the phrase "be in the air" to express that a situation is felt by many people the same way .
Example:
Desperation was in the air in the last minutes of the match. They had to score a goal or they were out of the competiton.

We use the phrase "be up in the air" to express that something is uncertain , undecided or unresolved .
Example:
The future of the project is still up in the air.
In the pictures below, you will find some more examples that will help you to better understand how to use these two phrases.

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