A ufo?

I
I saw a tiny silver thread high in the air late yesterday afternoon. I shot it with 300 mm. This is what it looked like (cropped and original). (Of course I don't really know how high this was).

http://www.shuttertalk.com/forums/at...t.php?aid=1511
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http://www.shuttertalk.com/forums/at...t.php?aid=1512
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This airliner was nearby. I include it for comparison.
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http://www.shuttertalk.com/forums/at...t.php?aid=1513

A UFO?

I believe the standard response is "weather balloon"

It's an odd shape for a weather balloon. But that may be what it is.