The chief delayed effect is the creation of huge amounts of radioactive material with long lifetimes (half-lifes ranging from days to millennia). The primary source of these products is the debris left from fission reactions. A potentially significant secondary source is neutron capture by non-radioactive isotopes both within the bomb and in the outside environment.

When atoms fission they can split in some 40 different ways, producing a mix of about 80 different isotopes. These isotopes vary widely in stability, some our completely stable while others undergo radioactive decay with half-lifes of fractions of a second. The decaying isotopes may themselves form stable or unstable daughter isotopes. The mixture thus quickly becomes even more complex, some 300 different isotopes of 36 elements have been identified in fission products.

Short-lived isotopes release their decay energy rapidly, creating intense radiation fields that also decline quickly. Long-lived isotopes release energy over long periods of time, creating radiation that is much less intense but more persistent. Fission products thus initially have a very high level of radiation that declines quickly, but as the intensity of radiation drops, so does the rate of decline.

A useful rule-of-thumb is the "rule of sevens". This rule states that for every seven-fold increase in time following a fission detonation (starting at or after 1 hour), the radiation intensity decreases by a factor of 10. Thus after 7 hours, the residual fission radioactivity declines 90%, to one-tenth its level of 1 hour. After 7*7 hours (49 hours, approx. 2 days), the level drops again by 90%. After 7*2 days (2 weeks) it drops a further 90%; and so on for 14 weeks. The rule is accurate to 25% for the first two weeks, and is accurate to a factor of two for the first six months. After 6 months, the rate of decline becomes much more rapid. The rule of sevens corresponds to an approximate t^-1.2 scaling relationship.

These radioactive products are most hazardous when they settle to the ground as "fallout". The rate at which fallout settles depends very strongly on the altitude at which the explosion occurs, and to a lesser extent on the size of the explosion.

If the explosion is a true air-burst (the fireball does not touch the ground), when the vaporized radioactive products cool enough to condense and solidify, they will do so to form microscopic particles. These particles are mostly lifted high into the atmosphere by the rising fireball, although significant amounts are deposited in the lower atmosphere by mixing that occurs due to convective circulation within the fireball. The larger the explosion, the higher and faster the fallout is lofted, and the smaller the proportion that is deposited in the lower atmosphere. For explosions with yields of 100 kt or less, the fireball does not rise abve the troposphere where precipitation occurs. All of this fallout will thus be brought to the ground by weather processes within months at most (usually much faster). In the megaton range, the fireball rises so high that it enters the stratosphere. The stratosphere is dry, and no weather processes exist there to bring fallout down quickly. Small fallout particles will descend over a period of months or years. Such long-delayed fallout has lost most of its hazard by the time it comes down, and will be distributed on a global scale. As yields increase above 100 kt, progressively more and more of the total fallout is injected into the stratosphere.

An explosion closer to the ground (close enough for the fireball to touch) sucks large amounts of dirt into the fireball. The dirt usually does not vaporize, and if it does, there is so much of it that it forms large particles. The radioactive isotopes are deposited on soil particles, which can fall quickly to earth. Fallout is deposited over a time span of minutes to days, creating downwind contamination both nearby and thousands of kilometers away. The most intense radiation is created by nearby fallout, because it is more densely deposited, and because short-lived isotopes haven't decayed yet. Weather conditions can affect this considerably of course. In particular, rainfall can "rain out" fallout to create very intense localized concentrations. Both external exposure to penetrating radiation, and internal exposure (ingestion of radioactive material) pose serious health risks.

Explosions close to the ground that do not touch it can still generate substantial hazards immediately below the burst point by neutron-activation. Neutrons absorbed by the soil can generate considerable radiation for several hours.

The megaton class weapons that were developed in the US and USSR during the fifties and sixties have been largely retired, being replaced with much smaller yield warheads. The yield of a modern strategic warhead is, with few exceptions, now typically in the range of 200-750 kt. Recent work with sophisticated climate models has shown that this reduction in yield results in a much larger proportion of the fallout being deposited in the lower atmosphere, and a much faster and more intense deposition of fallout than had been assumed in studies made during the sixties and seventies. The reduction in aggregate strategic arsenal yield that occurred when high yield weapons were retired in favor of more numerous lower yield weapons has actually increased the fallout risk.

Effects on the Atmosphere and Climate

Although not as directly deadly as fallout, other environmental effects can be quite harmful.

Harm to the Ozone Layer

The high temperatures of the nuclear fireball, followed by rapid expansion and cooling, cause large amounts of nitrogen oxides to form from the oxygen and nitrogen in the atmosphere (very similar to what happens in combustion engines). Each megaton of yield will produce some 5000 tons of nitrogen oxides. The rising fireball of a high kiloton or megaton range warhead will carry these nitric oxides well up into the stratosphere, where they can reach the ozone layer. A series of large atmospheric explosions could significantly deplete the ozone layer. The high yield tests in the fifties and sixties probably did cause significant depletion, but the ozone measurements made at the time were too limited to pick up the expected changes out of natural variations.

Nuclear Winter

The famous TTAPS (Turco, Toon, Ackerman, Pollack, and Sagan) proposal regarding a potential "nuclear winter" is another possible occurrence. This effect is caused by the absorption of sunlight when large amounts of soot are injected into the atmosphere by the widespread burning of cities and petroleum stocks destroyed in a nuclear attack.

Similar events have been observed naturally when large volcanic eruptions have injected large amounts of dust into the atmosphere. The Tambora eruption of 1815 (the largest volcanic eruption in recent history) was followed by "the year without summer" in 1816, the coldest year in the last few centuries.

Soot is far more efficient in absorbing light than volcanic dust, and soot particles are small and hydrophobic and thus tend not to settle or wash out as easily.

Although the initial TTAPS study was met with significant skepticism and criticism, later and more sophisticated work by researchers around the world have confirmed it in all essential details. These studies predict that the amount of soot that would be produced by burning most of the major cities in the US and USSR would severly disrupt climate on a world-wide basis. The major effect would be a rapid and drastic reduction in global temperature, especially over land. All recent studies indicate that if large scale nucelar attack occur against urban or petrochemical targets, average temperature reductions of at least 10 degrees C would occur lasting many months. This level of cooling far exceeds any that has been observed in recorded history, and is comparable to that of a full scale ice age. In areas downwind from attack sites, the cooling can reach 35 degrees C. It is probable that no large scale temperature excursion of this size has occurred in 65 million years.

Smaller attacks would create reduced effects of course. But it has been pointed out that most of the world's food crops are subtropical plants that would have dramatic drops in productivity if an average temperature drop of even one degree were to occur for even a short time during the growing season. Since the world maintains a stored food supply equal to only a few months of consumption, a war during the Northern Hemisphere spring or summer could still cause deadly starvation around the globe from this effect alone even if it only produced a mild "nuclear autumn".

Information provided by: http://www.fas.org