| MadSci Network: Physics |
There are several ways that explosions cause damage. Some of them can be avoided, but others are only lessened by distance.
The first effect one probably thinks of is the blast. So-called "high" explosions travel outward faster than the speed of sound (about 330 m/sec), pushing the air ahead of them in a shock wave. The more powerful the explosive, the faster the shock wave moves. Shock wave velocity is typically 7000 to 9000 m/sec. You hear the explosion when the shock wave reaches you. If you're too close to the explosion, the shock wave will injure or kill you. If you can have a few seconds to get indoors after you see the blast, you might conceivably lessen your injuries. "Low" explosions, such as are caused by gunpowder, are subsonic, and so you can use the 330 m/sec velocity, which means about 5 sec/mile, to tell how far away they are, if you can see the explosion before you hear it.
The second way is by thermal radiation. This happens at the speed of light, so there's no warning. Shadows can be burned into objects and skin, as seen in photographs from Hiroshima and Nagasaki.
The third way is by heated air. An explosion sufficiently violent will form an out-moving fireball, moving at the speed of the explosion. Conceivably you might have a little time to know about it in advance, but it's doubtful that would do much good!
A fourth way, as you mention, is shrapnel. That's the most survivable, because you may have many seconds before it reaches you. Get behind something heavy if you can, or, at the very least, lay face-down on the ground, to minimize the effects of the horizontal component of the shrapnel's velocity.
With nuclear bombs, there is also the high-energy (ultraviolet, x-ray, gamma ray) radiation that happens at the instant of explosion, plus the fallout afterwards. There's nothing you can do about the first one, but it can take hours before the fallout arrives, so obviously you'd want to get as far away as possible, away from where the wind may be blowing it to.
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