Why, every Tuesday at 10:00, is there a beep & message on radio about announcement from "emergency broadcast" system. Who is this and why do we have this?

I love this segment. Chicago born and raised in the for three general I loved hearing how transplants love our city. Black dirt, I took for granted. Chicago history you betcha, my pops was a building inspector and gave the occupancy permit to the Sears Tower. What you talking about Willis?! Or food. Yep some of the best in the world. Tks, Kathie