这个问题很有意思，故也参与讨论，但不一定对，只是给网友们提供点参考。

张老师说：“......前者则通常称为条件意义的程度状语从句，后者为主句。”

的确如此，前面的“the + 比较级结构”就是状语从句，后面的“the + 比较级结构”是主句，两个 the 都是副词。zzwen 网友问：既然 The hotter the air is 是状语从句，为什么没有连词呢? 这个问题问得很好，但值得注意的是，这个状语从句是有连词的，前句中的 the 就是用作连词，意义为“如果到达某个程度”（引导有条件意义的程度状语从句），后句中的 the 是程度副词“达到这个程度”。张道真《英语语法大全》（2014年1月版P. 302）专门谈到这个问题，书中将这种状语从句归于“依据状语从句”的另一特殊类，摘录如下：

The (= if to a certain degree) (用作连词) poorer he becomes, the (= to that degree) (用作副词) less friends he has.

The more I earn, the less content I feel.

The farther I went, the lonelier I felt.

The more he studied music, the less he could (or could he) understand it.

The faster (that) you progress, the faster you will (or will you) retreat (可加上一个 that)

The more (that) you know of a profession, the more you like (or do you like) it.

The sooner (that) we start, the better it is (不是 is it).

The younger is the pupil, the naughtier he is.

The more I read, the more I travel, the more I think, － the more ignorant do I feel.

The sooner, the better. (没有主语和谓语动词)

The more haste, the less speed.

间或从句放在后面：

I love her more (and more), the longer we are separated. (= The longer we are separated, the more I love her.)

He became more (and more) stupid, the older he grew.