Duck and Cover

"Duck and cover" is a method of personal protection against the effects of a nuclear explosion. Ducking and covering is useful at conferring a degree of protection to personnel situated outside the radius of the nuclear fireball but still within sufficient range of the nuclear explosion that standing upright and un-covered is likely to cause serious injury or death. In the most literal interpretation, the focus of the maneuver is primarily on protective actions one can take during the first few crucial seconds-to-minutes after the event, while the film by the same name and a full encompassing of the advice, also caters to providing protection up to weeks after the event.

The countermeasure is intended as an alternative to the more effective target/citywide emergency evacuation when the latter would no longer be viable due to time constraints. Maneuvers similar, but not identical, to Duck and Cover also find use during the event of a sudden earthquake or tornado when preventive emergency evacuation is not an option, again, due to time constraints. In these cases, Drop, Cover and Hold on can likewise prevent injury or death which may otherwise occur if no other safety measures are taken.

As a countermeasure to the lethal effects of nuclear explosions, it is effective in both the event of a surprise nuclear attack, and during a nuclear attack of which the public has received some warning, which would likely be about a few minutes prior to the nuclear weapon arriving.

Immediately after one sees the first flash of intense heat and light of the developing nuclear fireball, one should stop, get under some cover and Drop/Duck to the ground. There, one should assume a prone-like position, lying face-down, and to afford protection against the continuing heat of the explosion further cover exposed skin and the back of one's head with one's clothes; or, if no excess cover or cloth is available, one should cover the back of their head and neck with their hands.

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Wikipedia