articles - "Half of the" vs "half of"

1. 
   

   Over half of the domestic flights were delayed because of bad weather condition.

   
   


2. 
   
   

   Over half of domestic flights were delayed because of bad weather condition.

   
   

First, Is there any difference b/w the use of "half of" and "half of the"?

Second, does it affect the use of singular/plural if we use "half of domestic flights" instead of "half of the domestic flights"?