|Quoting Aeroguy (Reply 12):|
A plane doesn't necessarily need to be supersonic to have that vapor cloud form.
Although I agree with thiscomment, the November issue of "Air & Space" also carries a photograph of an F/A-8 surrounded by a vapor cloud carrying the quote that "air pressure changes. combined with the right humitidy levels, result in a condensation clouds as this F/A-18 passes through the sound barrier"
Now one considers Smithsonian's "Air & Space" to be a reputable member of the aerospace press. I guess one could say that it "is the right humidity levels" which generates such a spectacle for all aviation buffs.