Blue Sky Days

The Beijing municipal government has set a target of 245 "blue sky days" for 2007. I'm not entirely sure what constitutes one of these, is it just when the sky is blue or does the air quality have to be within certain parameters? With 1000 new cars on the streets every day I don't fancy <del>there</del> (and me an English teacher, hah) their chances. More power to the bike people.