1.in the air在空中（地球周围较近的天空如空气中、气氛、气息等）；在流传中；正在酝酿中；悬而未决
in the sky在天空，这个天空一般只有太阳、月亮星星闪电等比较远的天空
There's a sky in the air.空中有一只风筝。
She begins dancing, kicking her legs high in the air.
她开始跳舞，做了空中高踢腿动作。
He threw the baby high in the air and it stopped crying.
他把孩子向空中高高抛起，孩子就停止了哭闹。
There is a plane in the air.
天上有架飞机。
Smoke hung in the air above the city.
城市上空烟雾弥漫。
The pilot has logged 1 000 hours in the air.
这位飞行员有1000小时的飞行记录。
The troops fired blanks in the air.
部队向空中放空弹。
In the new year there is such a feeling of joy in the air.
新年里，空气中有洋溢着欢乐的感觉。
Spring is here and romance is in the air.
春天来了，到处洋溢着爱情的气息。
It's Valentine's Day and lurve is in the air.
今天是情人节，空气中弥漫着爱情的气息。
Falling leaves, cool breeze, blue sky. Fall is in the air!
落叶，清风，蓝天。空气中弥漫着秋的气息！
Her laughter soon dissipated the tension in the air.
她的笑声很快消除了紧张气氛。
After he finished his speech, I felt a sense of tension in the air.
他讲完话后，我感到空气中有一种紧张感。
There's romance in the air.
有种浪漫的气氛。
The lights were low and romance was in the air.
灯光暗淡，弥漫着浪漫情调。
The stench of treachery hung in the air.
到处都是可耻的叛变的气息。
He could taste the salt in the air as he got nearer to the coast.
当他靠近海岸时，他能尝到空气中的盐味。
The program is still in the air.这个项目还没定下来。
Our travel plans are still up in the air.
我们的旅行计划尚未决定。
He told reporters today that the president's trip to Moscow is up in the air.
今天他告诉记者们总统是否前往莫斯科尚未确定。
With his contract expiring during the summer, the defender's future is up in the air.
由于他的合同将在夏天到期，这位后卫的前途未卜。
Something unspoken hung in the air between them.
他们之间有些心照不宣的事还没有解决。
She walked away and left the question hanging in the air.
她走开了，将疑问留在人们心里。
The rumors are in the air that he has been fired.有人说他被辞退了。
in the open air在户外，在野外
例如，
Children tend to play in the open air.孩子们一般都在户外玩。
After a day’s work, it's necessary to get the kids out in the open air for a while.
学习了一天之后，有必要让孩子们在户外活动一会儿。
Last Summer, we went to the mountains. We slept out under the stars, and ate our meals in the open air.
去年夏天，我们去了山里。我们幕天席地，风餐露宿。
但是，空中楼阁只能说：
Their conceptions have all been proved to be castles in the air.
他们的想法都被证明是空中楼阁。
异想天开只能说：
This talk of moving to Australia is all just pie in the sky.
移居澳大利亚之说纯属异想天开。
He can't help thinking it's all just 'pie in the sky' talk.
他禁不住想所有这些不过是“画饼充饥”的空话而已。
The true regeneration of devastated Docklands seemed like pie in the sky
遭毁坏码头区的真正重建看起来希望渺茫。
The sun was high in the sky, blazing down on us
太阳高挂在空中，火辣辣地照在我们身上。
The sun was low in the sky.
太阳低挂在天空。
The sun is already high in the sky.
已经日上三竿。
A bright moon is shining in the sky.
一轮明月当空照。
A full moon hung in the sky.
一轮圆月悬在空中。
The stars were points of light in the sky.
天空中的点点光亮就是星星。
Sirius is the brightest star in the sky.
天狼星是天空中最亮的一颗星。
Stars twinkled in the sky.
星星在天空中闪烁。
Islands are scattered all along China's coastline like stars in the sky.
中国沿海岛屿星罗棋布。
Thunder crashed in the sky.
霹雳在空中炸响。
Thunder boomed in the sky overhead.
雷声在天空中隆隆作响。
Thunder cracked in the sky
空中雷声咔嚓作响。
Several people described seeing strange lights in the sky.
好几个人都说看到天上出现了奇异光芒。
The dark clouds in the sky meant that a storm was coming.
天空中的乌云预示暴风雨即将来临。
Clouds are floating in the sky.
云朵飘浮在空中。
A white cloud floated in the sky.
天空中飘浮着一朵白云。
Look, a plane is flying in the sky.
瞧，一架飞机在天空飞翔！
There are some birds flying in the sky.
天空中有几只鸟儿在飞翔。
throw one's hands up in the air 表示某人对做某件事失去信心，表示经过再三努力后仍未成功，因而认输或放弃了，例如：
When he saw the pile of work assigned by his supervisor, he threw up his hands up in the air.
当他看到主管分配给他的一大堆工作时，他放弃了。
2.on the air （用无线电、电视、手机）播送，广播，
We went on the air, live, at 7.30.
我们上了7点30分进行的现场直播。
A new show will be on the air in two weeks.
一个新节目将在两周后播出。
It's her first time to go on the air as presenter.
这是她第一次以主持人的身份上电视。
She is going on the air as presenter of a new show
她即将上电视主持一档新节目。
His show is on the air at 6:00 tonight.
他的节目今晚6点播出。
Rockwell hopes the program can be on the air within a year
罗克韦尔希望该节目能在一年内播出。
They give a slant to every single news item that's put on the air.
他们播放的每一则新闻都有所偏向。
She will be on the air next week.
她下周开始广播。
The popular television series Friends was on the air for ten years before it finally ended.
广受欢迎的电视连续剧《老友记》在播出十年后终于落幕了。
3.by air乘飞机，相当于by plane
Many people travel to different places in the world by air.
许多人乘飞机到世界各地旅行。
It is generally not advisable for expectant mothers to travel by air after the 28th week of pregnancy.
怀孕28周以后的准妈妈一般不宜乘坐飞机出行。
Children under five are not allowed to travel by air alone.
五岁以下的儿童不允许单独乘飞机旅行。
When you’re travelling, which do you prefer, by train or by air?
当你旅行时，你喜欢坐火车还是坐飞机？
Casualties had to be brought to hospital by air.
伤者不得不用飞机送往医院。
He was the first man to navigate the Atlantic by air.
他是第一个飞越大西洋的人。
She left by plane for Berlin.
她乘飞机去柏林了。
You can get there just as cheaply by plane.
你坐飞机到那儿同样便宜。
Flown by plane across the country just to deliver important news in person?
乘坐飞机跨越全国只为亲自传达重要的消息?