网友好，这个问题网上有讨论。我粘贴一个回答过来供参考。更多见：https://ell.stackexchange.com/questions/20131/what-does-this-mean-going-to-be-something-doing-in-the-humidity-line-to-nigh?rq=1

下面内容，版权归StoneyB 先生。

This is a jocular statement in early 20th-century US colloquial diction.

1.      The subject has been omitted; this is a matter of conversational deletion.

The omitted subject is the dummy or existential there, so the matrix clause is ‘There is going to be something doing.’

2.      There is something doing ... This is a stock phrase meaning approximately Something is happening or There will be activity or (in 1960s slang) Something is going down.

3.      In the X line ... Originally line in uses of this sort meant the ‘right line’ to be followed in any activity, but by the late 18th century the sense was extended to the line of business or occupation in which one engages, or the line of merchandise sold by a particular salesman, and was eventually extended in jocular use to what you observe here: a deliberately ponderous way of saying having to do with X.

So what we have is

There is going to be considerable activity having to do with humidity tonight.

In other words, “The weather is going to be very humid tonight.”