What's meaning of ”Lee waves are standing waves resulting from flow over a hill somewhere upstream ”
“Lee waves are standing waves resulting from flow over a hill ” until here i can totally understand， but i don’t know what is “somewhere upstream”means. i understand the words meaning, but i can't understand its meaning in the sentence.

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Answer

'Stream' refers to flow of a liquid. 'Upstream' refers to somewhere closer to where the fluid comes from. Downstream refers to further away from that source. 
'Lee' is downstream of some object in the flow. That is the meaning of the word 'lee'.
What may not be obvious here is that they are referring to wind. The 'fluid' is air.
So - if you have a northerly wind - the wind is coming from the north. The hill must be to the north of where the waves are forming. The wind comes from the north, flows over the hill, then forms waves to the south of the hill.

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