Just seen this and reminds me of what my Dad told me about the bombing of Sheffield. He was there. Bear with me as I may go on a bit. The Germans knew Sheffield as "the city on wheels" because they couldn't find it, hard to believe I know. One of the reasons was because whenever they were on their way, the factories would begin belching out Black smoke which, due to the topography of the city, would shroud the place in smog, resulting in it being hidden. My dad lived at Wisewood and they always knew the Germans were heading this way because the Loxley Valley works would begin producing black smoke which would then drift down the valley and over the city. This was before the sirens sounded. More often than not, they were heading for Manchester or Liverpool but that wasn't known at the time.
If you look at the map of the bombs, it gives a clue as to how they eventually found Sheffield..........they followed the railway tracks up from Derby which explains the first bombs hitting Millhouses, the city centre getting the brunt of the attack and the heavy industry of the east end being pretty much missed completely. It was always fascinating to chat to my dad about the war and he remembered the Sheffield blitz clearly, the devastation it brought to families who lived in the city but the total lack of effect it had on Sheffields ability to keep producing steel for the war effort.