on the alert

注意, 提防, 警戒, 警惕着, 处于戒备状态

例句与用法：

A feather floats about on the air, especially on a windy day. If there is a shower of rain, the feather will get wet and heavy with water, and then it will sink straight to the ground.

羽毛会在空中四处飘动，尤其是在多风的天气。如果遇上阵雨，就会因为淋上雨水变湿就沉，然后就会垂直落回到地上。

The bombers swooped on the air base.

轰炸机突袭空军基地。

This channel comes on the air every morning at 7 am.

这个频道每天早晨7点钟开始播放。

The news will be on the air tonight.

这个新闻今晚将要广播。

详细释义：

警戒着; 随时准备着

They have put the army on the alert.

他们已使军队进入戒备状态。