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Some people say Ushaia or Puerto Williams for being the southernmost cities of Argentina and Chile respectively, but I think some cities located a bit more up north, but in more inner parts of Patagonia (which means they have a more continental climate) would be better candidates to be the coldest, like Rio Gallegos or Comodoro Rivadavia.
Sarmiento (Chubut) which is not a city but a town, has the lowest temperature record registered in all south America. Almost -33 degrees Celsius.
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