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

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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.