How much time did people have to take shelter during the Blitz in 1940-41?

Question

Birmingham: In a BBC History article, An Air Raid Incident from World War Two relating the experiences of a Birmingham teenager during the Birmingham Blitz, the writer says (my highlighting):

As soon as the siren sounded, Daddy would prepare to leave the house taking with him some cushions and the little lamp, plus a few odds and ends. Mother would then make a flask of tea or coffee and sometimes soup, some sandwiches and a biscuit tin full of cookies.

This family, at least, doesn't seem to have been in too much of a hurry to take shelter (even if the water was already boiling). Note also the use of the habitual 'would', implying predictability.

London: From what I've read about the London Blitz (7 September 1940 – 11 May 1941), there is little information on how much time people had to take shelter in London. Based on what my mother has told me - she was in London (near Lord's cricket ground) during part of the Blitz - the time between the siren going off and the first bombs falling was very inconsistent. However, my mother was only 8 or 9 at the time, it was a long time ago, and she wasn't in London for the whole duration of the Blitz so she doesn't know all the details.

---

Air raids (and thus sirens) could be on and off throughout the night and the warning systems were inefficient in London (though presumably they improved after a time), so it seems reasonable to conclude that predicting how much time one had to take shelter in London was difficult, at best. Also, there were apparently two warning sirens for "people doing important war work" so the 'rules' weren't the same for everyone.

In Birmingham, however, the only piece of evidence I've been able to dig up seems to imply that the time one had to take shelter was more predictable.

My questions:

1. Can anyone confirm that, in London during the Blitz (7 September 1940 – 11 May 1941), the time one had to take shelter once the air raid siren went off was unpredictable ?

2. In Birmingham, was this time more predictable? If so, is it possible to say approximately how much time people had?

---

Note: I would also be interested in warning times in Liverpool, Plymouth or Exeter if anyone has any information on these cities. The time period, though, should be 1940-41 and I'm not asking about the 1945 V-2 rocket as it is common knowledge that this gave no warning.

Answer 1

More of a guess than the actual answer...

One data point is the planes being used. The German bombers had a top flight speed ranging from around 300km/h to around 500km/h depending on the model. They were escorted by fighters and there could be dogfights before they reached their targets, so it's probably sane to think of that as a maximum approach speed.

Another data point is the extent of the radar coverage. If the map is anything to go by, and depending on the point in time, aircraft would get detected around 150km off the coast at best, and when it's nearly overland at worst.

Put another way, in the rosiest possible circumstances - that is, undetected until near UK soil, and flying full speed ahead with no RAF reception committee - a Luftwaffe bomber could theoretically be over London in a dozen minutes or so and over Birmingham in about an hour. In practice, radar would detect the aircraft a half hour earlier in both cases and the RAF would seek to intercept the Luftwaffe.

There also is a practical concern, which introduces another big unknown in this back of the envelope calculation. Namely, how long did it take for military staff to trigger the alarm systems in this or that location - after all, there's little point in having sirens run in more distant cities like Birmingham until it's established that they're a potential target.