Meteorology and the Tests
Whenever any kind of explosion takes place, a very high temperature is produced in a very short time in a confined space. If a mass of air is heated, it expands and if it is heated very rapidly by an explosion, it expands very quickly. This rapid expansion causes a high pressure to be built up and the release of this high pressure is felt as a shock wave accompanying the explosion. The shock wave expands and produces considerable damage due to blast and is, of course, the reason why a thing which is exploded is often said to be " blown up".
In a tiny fraction of a second after an atomic bomb is exploded, the temperature produced is many million degrees Centigrade. The temperature of a really hot furnace is about
l000°C. so by comparison we see that the temperature which has been produced is fantastically high. This extremely hot source produces intense radiation and the first effect of this is a flash of light so intense that anyone looking at it would temporarily be blinded. A good deal of the radiation is absorbed by the air immediately around the explosion which becomes so hot that it is incandescent and forms the "fire-ball". The fire-ball increases rapidly in size, heating the air as it does so and, in addition, an intense shock wave is produced which gives the blast effects.
The meteorologists are not very interested in the blast effects, but they and the safety officers are vitally interested in the fire-ball and what it produces. It is well known that hot air rises, simply because the heat has caused it to expand and it becomes light and buoyant. At about
I0 to 15 seconds after the explosion, the fire-ball will begin to rise, cooling, expanding and losing brilliancy as it does so, and carrying with it most of the products of the bomb. The intense heat of the explosion causes the material of which the bomb is made to be vaporized and later this vapour cools and condenses into small solid pieces of radio-active (RA.) material.
As the cloud forces its way upward, still expanding, it takes in air from the surrounding atmosphere and some of the cloud gets left behind. The whole thing then resembles a gigantic mushroom with a head (the cloud) and a stalk (the stem).
If the explosion has been high in the air, then the fire-ball will not touch the ground or sea and the only RA. materials in the rising cloud are the tiny bits of the bomb. These are so small that they are carried to great heights and fall so slowly that they take weeks or even months to reach the earth. By that time their activity will have decayed and in any case they will be very diffuse and will not be dangerous.
From the point of view of fallout a burst high in the air differs greatly