what does "strike flat"mean?
The wind had been high overhead. The rain was a solid wall, from sky to earth. Jody struck it flat, as though he had dived against it from a great height. It hurled him back and threw him off his balance.

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Answer

It mean it "hit it head on" or in other words, he was facing the wind and was struck by its full force.

Answer

Imagine rain falling straight down. Only your hair and your shoulders would get very wet.

This was different.

The wind-driven rain suddenly hit Jody all at once. It hit him everywhere on the front of his body--face, chest, arms, legs. It hit them all at the same instant. All that force suddenly hitting all of that area "hurled him back."

I think "dived" means what U.S. speakers call a "bellywhopper," where the entire front of your body hits the water all at once.

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