As the brilliant fireball rises in the sky, it draws up a vast amount of earth that is melted or vaporized and contaminated by the radioactive residue of the explosion. A little later this material, condensing in the cold upper air like rain or snow, starts falling back to earth because, like ash from a fire, it is heavier than air. It is called fallout because it falls out of the sky, wherever the winds may blow it. You cannot tell from the ground which way it will be carried because its scatter is determined by high-altitude winds, which may be blowing in a different direction from the ground-level winds you can observe. About five miles from the explosion, the heavier particles – early fallout – would reach the ground in half an hour. Twenty miles away, people may have nearly an hour to get ready. One hundred miles away the fallout may not start for four to six hours. All this early fallout, which carries the bulk of the radiation danger, descends in less than 24 hours. The less dangerous lighter particles – delayed fallout – might stay aloft for months.