The northern part of the midwest is referred to as the “rust belt” because of its cold temperatures and generous precipitation

That's not where the term came from.  "Rust Belt" is a term
for the region straddling the upper Northeastern United States,
the Great Lakes, and the Midwest States, referring to economic
decline, population loss, and urban decay due to the shrinking
of its once powerful industrial sector.

The answer is that they depend on rainfall for their crops to grow. hope this .

Iwould say the answer is: trade windshope that : )