Air raid sirens were used during World War II to alert the public of an imminent air raid so that people could take shelter.
Mechanical sirens (either electrically powered or operated via a hand crank) operate as follows: a motor or crank turns either one or two (for single and dual tone sirens, respectively)fans or impellers. (In the case of a dual tone siren, the two impellers would have different numbers of blades, often 5 and 6 or 10 and 12, equally spaced). The impeller(s) would be placed inside a housing with slots that corresponded to the number of blades. As the impeller turned, it would draw air in and force it out the slots, creating a tone. As the speed increased, so did the frequency of the note produced. In the case of a dual tone siren, the ratio of the number of blades was equal to the ratio of the note frequencies; a 5:6 ratio would produce a minor third (the most common type), a 4:5 ratio would produce a major third, a 2:3 ratio would produce a major fifth, and a 1:2 ratio would produce an octave.
The siren would be used to inform the public of impending doom. In the case of an air raid, a "red alert" would be sounded. A red alert consisted of an oscillating tone, or slow rising and falling wail. This oscillation was achieved by turning the siren on, letting it rise to the highest note, and then turning the siren off, allowing the blades to slow and the pitch of the note to drop. The siren was then restarted and the process repeated.
When the danger passed, the all clear, or "white alert" would sound. A white alert consisted of the siren being turned on, and letting it stay on the high note for an extended length of time.
During the Cold War, sirens came back into use. There was, however, an added degree of complexity: the danger of radioactive fallout. In order to warn the public of possible fallout (usually within the hour), a "grey alert" was sounded. A grey alert was produced by having the siren hold the high note for 2 1/2 minutes, while opening and closing a shutter at the front of the siren. This would cause an intermittent high note. On hand-operated sirens, the shutter was to be open for five turns and closed for five turns. If there was an immediate danger of fallout, a "black alert" was sounded. Instead of the evenly spaced high notes of the grey alert, a black alert would sound the Morse code "D", or long short short. Another variation of the black alert would involve three short notes instead of the Morse code D.
Electronic sirens produce the same types of sounds that mechanical sirens could, but they do it using speakers as opposed to impellers. Electronic sirens have the added advantage of (depending on the model) being able to double as PA systems.
Now that there is no danger (supposedly) of air raids or fallout, alert sirens are often used to warn of natural disasters (such as tornadoes, earthquakes, hurricanes, fires, etc.), chemical spills, terrorist attacks, or in such enlightened places as Chicago's suburbs, they warn of snowplows being deployed on the streets (because, of course, snowplows present a grave threat to the public safety).
Credit to: http://freespace.virgin.net/roy.smith5/sirens.htm, http://www.cityofchicago.org/Emergency911/Tornado.html
Suburbs that use sirens to warn of snowplows: Skokie (http://www.skokie.org/public/snow.html)