Mark Perry Jr., age 12, of Utica, N.Y., for his question:

HOW IS FALLOUT PRODUCED?

Fallout is radioactive material that settles over the earth after an atomic or hydrogen bomb explodes. The effects of fallout depend on the kind of radioactivity involved and the length of exposure to it.

All nuclear explosions produce a great fireball of intensely hot gases. When the device explodes close to the earth's surface, this fireball may touch the ground. If it does, everything within the area that the fireball touches, including natural and man made objects, is vaporized or turned into a gas.

The fireball then starts to rise, carrying this vaporized material with it. As the fireball rises, a low pressure area forms under it.

Air rushes in to fill the partial vacuum of the low pressure area, carrying along with it dust, dirt and other small particles. Much of this debris may be lifted up through the atmosphere along with the fireball.

As the vaporized materials rise and cool, some of them condense into solid particles. Atoms of the various radioactive elements that are produced by the blast cling to these particles. These radioactive materials eventually return to the earth in the form of fallout.

Fallout particles range in size from fine invisible dust to ash of snowflake size. Fallout is described as either local or distant, depending on how far from the blast site it settles to the earth.

Local fallout consists of the larger and heavier particles that fall to earth within a few hundred miles of the blast site. The time it takes for these particles to reach the earth, and the distance they travel from the blast site, depend on their size, the altitude they reach, the winds that carry them and the latitude at which the explosion takes place.  Distant fallout may be scattered by winds to any part of the world.

Some of the distant fallout remains in the atmosphere for years before it settles. Winds traveling through the troposphere, or the lowest layer of the atmosphere, carry some distant fallout. But fallout particles produced by the most powerful nuclear explosions may rise as high as the stratosphere, or the layer of atmosphere above the troposphere.

Winds traveling at an altitude of about 50,000 feet carry most of the fallout in the troposphere. These winds move in a generally eastward direction. They make a complete circle around the earth in from one to two months.

Because of the winds, the fallout material from the troposphere may settle anywhere on a fairly narrow band around the earth near the latitude of the blast site. Fallout debris is carried down to earth from the troposphere by fog, rain or snow.

The fine fallout particles that reach the stratosphere settle back to earth slowly. The length of time they remain in the stratosphere varies from many months of many years.

An important factor affecting the fallout is the latitude at which the particles enter the stratosphere. Debris entering above the equator comes back to earth more slowly than debris entering above the poles.