Tim Bowen's selection of idioms will have you walking on air.
In the midst of the chaos caused by a volcano errupting, one newspaper led with the headline 'Travel plans are up in the air for at least a week'. In this sense, up in the air means that the travel plans were undecided or unresolved, although, in a literal sense, of course, the actual planes were clearly not in the air.
If something is simply in the air, it means that people all have a similar feeling, especially a feeling that something exciting or new is happening or about to happen, as in 'There was a feeling in the air that it was time for a change' or 'Spring is in the air'.
Things can appear out of thin air or disappear into thin air, meaning that they have appeared or disappeared in a sudden and mysterious way, as in ‘When I looked around, he seemed to have vanished into thin air'.
If you are feeling extremely happy or pleased with yourself for some reason, you might be walking on air or even floating on air.
If you clear the air, you discuss a problem or a difficult situation with someone in order to make things improve, as in ‘I think it’s time we cleared the air, don’t you?’
Hot air is something most often associated with politicians who are prone to making statements that sound impressive but are in reality neither sincere nor sensible.
A breath of fresh air is something or someone new, interesting and exciting, as in 'Wind farms could be a breath of fresh air for the power industry’ or ‘Mr. Brown has been a breath of fresh air in a team that had seemed to have lost its way’.