Blackout (wartime)

A blackout during war, or in preparation for an expected war, is the practice of collectively minimizing outdoor light, including upwardly directed (or reflected) light. This was done in the 20th century to prevent crews of enemy aircraft from being able to identify their targets by sight, for example during the London Blitz of 1940. In coastal regions a shore-side blackout of city lights also helped protect ships from being seen in silhouette against the shore by enemy submarines farther out at sea.

In actual warfare, according to M. R. D. Foot, blackouts do not impair navigation by bombers because navigators focused more on reflected bodies of water, railroad tracks, or large highways. The chief purpose is to mobilize the entire civilian population, and provide a test to make sure they are obeying the rules.

During World War II, the Air Ministry had forecast that Britain would suffer night air bombing attacks causing large numbers of civilian casualties and mass destruction. It was widely agreed that navigation and targeting would be more difficult if man-made lights on the ground could be extinguished. As early as July 1939, Public Information Leaflet No 2 (part of the Air Raid Precautions (A.R.P.) training literature) warned of the need for popular discipline to ensure that the blackout regulations were fully enforced during the blackout periods.

Blackout regulations were imposed on 1 September 1939, before the declaration of war. These required that all windows and doors should be covered at night with suitable material such as heavy curtains, cardboard or paint, to prevent the escape of any glimmer of light that might aid enemy aircraft. The Government ensured that the necessary materials were available. External lights such as street lights were switched off, or dimmed and shielded to deflect light downward. Essential lights such as traffic lights and vehicle headlights were fitted with slotted covers to deflect their beams downwards to the ground.

Shops and factories had particular problems. Factories with large areas of glass roofing found it impossible to install temporary blackout panels and permanent methods (such as paint) lost natural light during daylight. Shops had to install double "airlock" doors to avoid lights showing as customers arrived and departed.

Blackouts proved one of the more unpleasant aspects of the war, disrupting many civilian activities and causing widespread grumbling and lower morale.

The blackout was enforced by civilian ARP wardens who would ensure that no buildings allowed the slightest chink or glow of light. Offenders were liable to stringent legal penalties.

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Wikipedia