This feature was first revealed in a 8-minute long interview with fellow French JVN on September 28th, 2009. At first, this news was relieved by British website Gamezine (who didn't made any interview) but following the issue of IGN's own mini interview later the same day, Gamezine then claimed the weather conditions availability was unsure.
This rumor has now been smashed by Gameblog's latest interview which confirms the introduction of weather conditions in upcoming Gran Turismo 5.
