First of all let me just say that I am very glad for this forum. All of you people out here answering all these questions is just wonderful. I for one am very grateful.
Ok, on with my question.
Listening to ATC on my scanner, I sometimes here this: NWA 353 climb and maintain 17,000 and join J21. What is J21? I have heard some people say it has to do with the jet stream. If that is the case, can someone be a little more discriptive about what that means? Thank you in advance.