Looking on the internet deeply has found these results:

the rain became heavier is the most popular phrase on the web.

the rain became more heavier 10 results on the web	the rain became heavier 1944 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Apr 25, 2015 ... Then I headed up the main road as the rain became heavier. I stopped at a bakery for a bit and then at an outfitter store killing time, then I finally�... Aug 10, 2014 ... "They told us to stay in place on the 12th," Poulter said, noting that the rain became heavier at that point, making the fairway barely visible. Feb 19, 2015 ... ... found clusters of "superterminal" drops falling faster than they should based on their size and weight, especially as the rain became heavier. However, Audrey notes that as the rain became heavier, �ultimately no one heard us say them to each other besides us.� After the ceremony, guests sipped�...

the rain became more heavier

10 results on the web

the rain became heavier

1944 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Apr 25, 2015 ... Then I headed up the main road as the rain became heavier. I stopped at a bakery for a bit and then at an outfitter store killing time, then I finally�...
Aug 10, 2014 ... "They told us to stay in place on the 12th," Poulter said, noting that the rain became heavier at that point, making the fairway barely visible.
Feb 19, 2015 ... ... found clusters of "superterminal" drops falling faster than they should based on their size and weight, especially as the rain became heavier.
However, Audrey notes that as the rain became heavier, �ultimately no one heard us say them to each other besides us.� After the ceremony, guests sipped�...