Air Raids could last from just afew hours to a whole night.
Although mechanical sirens existed before the 1900's, Federal Sign and Signal Corp (then Federal electric) purchased the patents for an electronic siren in 1915. A common purchaser were coal miners...
Because they gave people some warning and therefore time to take shelter
The danger of being subjected to aerial bombardment .
it generally last 2 days the most if not popped if it hits a tree its dead:(