What is the change-of-state le 了?

As a modal particle, the sentence-end le 了 indicates that a new situation has emerged or a change has taken place. We often call this sentence-final modal particle 'change-of-state le 了' or 'new situation marker le 了'. Compare the following sentences. One has a le 了 at the end while the other does not. They convey different meanings.

1. 
   Tiānqi rè le.
   天气热了。
   The weather has become hot.
   
   This sentence implies that the weather was not hot before but now it has changed.


2. 
   Tiānqi hěn rè.
   天气很热。
   The weather is hot.
   
   Without the modal particle le 了, the sentence simply announces the fact that the weather is hot.