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 one's head and neck with one's hands.