Warmer air holds more water vapour.温暖的大气能容纳更多水蒸气

water vapour水蒸汽；水汽

Water vapour condenses to form clouds.

水蒸气凝结形成云。

When water changes to water vapour, we say it evaporates.

当水变成水蒸气时, 我们把这个过程叫做蒸发.

So, on average, our atmosphere is 4% more humid than it used to be 30 or 40 years ago.因此平均来讲，大气较三四十年前湿度增加了4%

on average平均

On average each report requires 1,000 hours to prepare.

准备一份报告平均需要1,000小时。

On average, American firms remain the most productive in the world. 总体来讲，美国公司在全球范围内还是产值最高。

used to（用于过去持续或经常发生的事）曾经

The Greeks used to worship several gods.

过去,希腊人崇奉好几种神。

They used to buy ten kilos of beef in one lump.

他们过去常买10公斤重的整块牛肉。