@exspeedbird - you said: "..."I cannot transmit an improvement in the visibility until it has been positively sustained for 10 minutes " he said.
......
Maybe the ATC controller was following the book, but my question later was - if the RVR was reducing below legal minima one dark and dirty night, would they not tell me until 10 minutes after I had crashed on the field ?"
Don't you answer your own question? Note that the key words: "I cannot transmit an improvement in the visibility until it has been positively sustained for 10 minutes" he said."