How big is an Atomic bombs explosion?

Answer 1

Depending on the yield and type of detonation, a nuclear weapon detonation can have a radius of several hundred feet to several hundred miles.

Nuclear weapons have been developed of the size as small as a small suitcase, to weighing several tons.

An example of a typical Cold War era weapon in the five megaton range with a surface burst would produce a blast crater approximately a mile and a half wide, and half a mile deep, with a blast radius of about fifty miles. Consider that for a moment, then consider that the Soviet Union at its height claimed to have developed a weapon with a selectable yield that could be as large as 100 megatons. They did test the device, air burst near the Arctic Circle in the 1950's, and the effects were felt several hundred miles away.

A little bit more about yield and burst:Nuclear weapons have yields of under 1 kiloton (the M-28/29 Davy Crockett Tactical Nuclear Recoiless Gun--also referred to sometimes as a "nuclear grenade"--circa 1955, fired the M388 warhead with a yield of .001 - .002 kiloton) to at least 50 megatons (the Soviet Tzar Bomba was said to have a yield of100 megatons). A one megaton device is relatively small but still can cause tremendous devastation. The bombs dropped on Hiroshima and Nagasaki were in the 20 kiloton range (Fat Man was larger, estimated at around 22 kiloton, Little Boy was estimated at around 15 kiloton).

During the height of the Cold War, much of the US would have been expected to receive missiles of at least 5 megaton payloads. A five megaton surface burst blast will leave a crater approximately 1½ miles across and ½ mile deep. The blast range of such a weapon would send a wave outward at about 250,000 feet per second (initially...quickly dropping to approximately the speed of sound) for a thirty to fifty mile radius, depending a bit on landforms, falling off in speed as it expanded outward of course.

This leads of course to the second element, burst:

There are three types of nuclear burst: air, surface, and subsurface. The type of burst that causes the greatest amount of physical damage is a surface burst device. This is a weapon that is designed to detonate at impact. The greater destructive force is caused by such close proximity to the ground, where the detonation is concentrated into the ground and ejects debris upward and out in the dramatic "inverted bowl" form that can be seen in much of the test detonation footage.

The second least physically devastating form, or potentially lessor physically devastating form is the air burst. An air burst detonation transmits the force of the blast equally around it. Depending on the altitude at which it fires, an air burst may cause great to no physical damage to people or structures on the ground. Again at the height of the cold war, it was estimated that two Hydrogen devices detonated at high altitude over the US would have caused an EMP (Electromagnetic Pulse) that would have fried all electronic devices in the US, Mexico, and much of Canada. However, even a small hydrogen device fired at lower altitude could create a blast wave that would essentially scrub the ground clean in the blast radius, but would leave no crater, or a minimal crater in its wake. This was the idea essentially behind the Neutron Bomb; one that when detonated left most structures intact outside of the immediate blast zone, but killed by the high radiation it released, leaving the territory essentially clean for occupying troops.

The least devastating form, or least readily visibly devastating form of blast is the sub-surface detonation. Sub-surface devices were developed for attacking underground, primarily "hardened" targets such as bunkers and silos. A small to medium subsurface burst will effectively destroy an enemy stronghold several hundred feet below ground but can leave the surface largely undisturbed. Larger detonations in the 100's of kiloton to the megaton range can leave large subduction zones where the surface above the detonation drops to fill in the void left by the detonation. This of course depends again upon the yield, and also the depth at which the detonation is set.

Answer 2

From measurements in Japan, a atomic bomb explosion can be from 2-7 miles in diameter.

Answer 3

Depends on the size of the bomb, and how high above ground it explodes.