The most immediate indication of a nuclear explosion is the intense light emanated from thefireball: just like seeing the flash of an electronic flash a million times more intense. The table on the left displays the flash duration as related to the weapon’s power up to 0.7 megaton.
After a lapse of time of 5 to 11 seconds, depending on the weapon’s power, the shock wavetraveling at a speed in excess of 350 kilometers per hour will follow creating enormous overpressures and afterwinds with winds which may be in excess of 1,000 kilometers per hour while the fireball rises in the atmosphere.
At the very moment that one becomes aware of a nuclear explosion, if the thermal radiation is survived, the response will have to be immediate and seek shelter: if in the open to lay prone on the ground in a longitudinal position in respect to the explosion so as to offer a minimal part of the body to the incoming shock wave, trying to shield the head with whatever available in order to protect it from the dangerous projectiles, i.e., the debris, within the shock wave.
Should the occasion arise it would be necessary to find shelter in the best way which the circumstances allow: in a culvert or a ditch, behind a low and thick wall, or whatever may protect from the shock wave and it will be necessary to remain in that position on the ground or screening agent for at least 2 minutes, roughly enough to be certain that the immediate effects of the explosion have subsided.
Less than 1
1 to 2
Sheltering inside whatever is not firmly grounded, i.e., inside or behind a car, it to be absolutely avoided even if the only choice would be just to lay prone on the ground. The distance from the source of the thermal radiation will be crucial: a 20 kiloton explosion’s initial thermal radiation will have no negative effects even if completely exposed at a distance of 1,700 meters; the same explosion will be lethal for 50% of the people exposed at 1,200 meters; 50 meters closer and 90% of those exposed will suffer the lethal effects of the initial thermal radiation and, 50 meters closer, hardly anyone would survive.
This margin of distance which makes the difference between life and death is due to the atmospheric absorbing average of the thermal radiation and hence it is of great importance in low yield explosions, like that in this example.
The yield of the bomb considered in the example above, i.e., 20 kiloton,will produce an overpressure of 5 psi (0.35 kilograms per square centimeter) at a distance of 1,600 meters; a 10 megaton weapon will produce the same effect at a distance of over 12 kilometers.
The overpressure of diverse weapon’s yield is given by a formula known as “law of cubic root”. We see, above, that a pressure of 5 psi is released by a 20 kiloton bomb at 16,000 meters while the same pressure obtains at over 12,000 meters with the explosion of a 10 megaton bomb. The values are obtained by extracting the cubic root of the ratio of the two weapons: 1 megaton is 500 times more powerful that 20 kiloton and the cubic root of 500 is approximately 8, hence 1.600 x 8 = 12,800.