They're not especially rare at these latitudes (I live in southern Finland, ~61°N). You see them fairly frequently on summer nights, rocket launches or not. But

They're not especially rare at these latitudes (I live in southern Finland, ~61°N). You see them fairly frequently on summer nights, rocket launches or not. But what is interesting that nobody seems to have written anything about them until the 20th century, which could indicate that suitable condensation nuclei did not exist in the upper atmosphere before humans started emitting them.