in the air与on the air

1. in the air 在空中；在流传中；渺茫。如：

Birds fly in the air. 鸟在空中飞。

There was thunder in the air. 天上打着雷。

The plan is quite (up) in the air. 计划还相当渺茫。

Look, hold it uP in the air like this. 看着，像这样把它举在空中。

2. on the air 在广播中。如：

The show is on the air. 演出正在转播。

I heard the news on the air. 我在广播中听到了这条消息。

What’s on the air this evening? 今晚有什么广播节目?

The show is on the air at seven o’clock. 这次演出七点钟播出。

(from www.nmet168.com)