Effects of Nuclear Weapons

The Fireball

The fireball, an extremely hot and highly luminous spherical mass of air and gaseous weapon residues, occurs within less than one millionth of one second of the weapon's detonation. Immediately after its formation, the fireball begins to grow in size, engulfing the surrounding air. This growth is accompanied by a decrease in temperature because of the accompanying increase in mass. At the same time the fireball rises, like a hot-air balloon. Within seven-tenth of one millisecond from detonation, the fireball from a 1-megaton weapon is about 134 meters across, and this increases to a maximum value of about 1.73 kilometers in 10 seconds. It is then rising at rate of 76 - 106 meters per second. After a minute, the fireball has cooled to such an extent that it no longer emits visible radiation. It has then risen roughly 7.2 kilometers from the point of burst.

Illustrated components of a nuclear explosion.

The Mushroom Cloud

As the fireball increases in size and cools, the vapors condense to form a cloud containing solid particles of the weapon debris, as well as many small drops of water derived from the air sucked into the rising fireball.

The early formation of the mushroom cloud.

Depending on the height of burst, a strong updraft with inflowing winds, called "afterwinds," are produced. These afterwinds can cause varying amount of dirt and debris to be sucked up from the earth's surface into the cloud. In an air burst with a moderate (or small) amount of dirt and debris drawn up into the cloud, only a relatively small proportion become contaminated with radioactivity. For a burst near the ground, however, large amounts of dirt and debris are drawn into the cloud during formation.

The color of cloud is initially red or reddish brown, due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools and condensation occurs, the color changes to white, mainly due to water droplets (as in an ordinary cloud)

The cloud consists chiefly or very small particles of radioactive fiision products and weapon residues, water droplets, and larger particles of dirt and debris carried up by the afterwinds.

The eventual height reached by the radioactive cloud depends upon the heat energy of the weapon and upon the atmospheric conditions. If the cloud reaches the tropopause, about 9.6 - 12.87 kilometer above the Earth's surface, there is a tendency for it to spread out. But if sufficient energy remains in the radioactive cloud at this height, a portion of it will ascend into the more stable air of the stratosphere.

The mushroom cloud forming at the Nevada Test Site.

The cloud attains its maximum height after about 10 minutes and is then said to be "stabilized". It continue to grow laterally, however, to produce the characteristic mushroom shape. The cloud may continue to be visible for about and hour or more before being dispersed by the winds into the surrounding atmosphere where it merges with natural clouds in the sky.

Thermal Pulse Effects

One of the important differences between a nuclear and conventional weapon is the large proportion of a nuclear explosion's energy that is released in the form of thermal energy. This energy is emitted from the fireball in two pulses. The first is quite short, and carries only about 1 percent of the energy; the second pulse is more significant and is of longer duration (up to 20 seconds).

The thermal pulse charring the paint

The energy from the thermal pulse can initiate fires in dry, flammable materials, such as dry leaves, grass, old newspaper, thin dark flammable fabrics, etc. The incendiary effect of the thermal pulse is also substantially affected by the later arrival of the blast wave, which usually blows out any flames that have already been kindled. However, smoldering material can reignite later.

The major incendiary effect of nuclear explosions is caused by the blast wave. Collapsed structures are much more vulnerable to fire than intact ones. The blast reduces many structures to piles of kindling, the many gaps opened in roofs and walls act as chimneys, gas lines are broken open, storage tanks for flammable materials are ruptured. The primary ignition sources appear to be flames and pilot lights in heating appliances (furnaces, water heaters, stoves, etc.). Smoldering material from the thermal pulse can be very effective at igniting leaking gas.
Thermal radiation damage depends very strongly on weather conditions. Cloud cover, smoke, or other obscuring material in the air can considerably reduce effective damage ranges versus clear air conditions.

The energy from the thermal pulse can initiate fires in dry, flammable materials, such as dry leaves, grass, old newspaper, thin dark flammable fabrics, etc. The incendiary effect of the thermal pulse is also substantially affected by the later arrival of the blast wave, which usually blows out any flames that have already been kindled. However, smoldering material can reignite later.

Effects of the thermal pulse on clothing

Thermal radiation also affects humans both directly - by flash burns on exposed skin - and indirectly - by fires started by the explosion.