USAAF – Air Attack Against the Japanese Home Islands

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The Air Attack Against the Japanese Home Islands

Basic United States strategy contemplated that the final decision in the Japanese war would be obtained by an invasion of the Japanese home islands. The long-range bombing offensive from the Marianas was initiated in November 1944, with that in mind as the primary objective. As in Europe prior to D-Day, the principal measure of success set for strategic air action was the extent to which it would weaken enemy capability and will to resist our amphibious forces at the time of landings. This led, originally, to somewhat greater emphasis on the selection of targets such as aircraft factories, arsenals, electronics plants, oil refineries, finished military goods destruction of which could be expected to weaken the capabilities of the Japanese armed forces to resist at the Kyushu beachheads in November 1945, than on the disruption of the more basic elements of Japan’s social, economic, and political fabric. Certain of the United States commanders and the representatives of the Survey who were called back from their investigations in Germany in early June 1945 for consultation stated their belief that, by the coordinated impact of blockade and direct air attack, Japan could be forced to surrender without invasion. The controlling opinion, however, was that any estimate of the effects of bombing on the Japanese social fabric and on the political decisions of those in control of Japan was bound to be so uncertain that target selection could safely be made only on the assumption that ground force invasion would be necessary to force capitulation.



With the benefit of hindsight, it appears that the twin objectives of surrender without invasion and reduction of Japan’s capacity and will to resist an invasion, should the first not succeed, called for basically the same type of attack. Japan had been critically wounded by military defeats, destruction of the bulk of her merchant fleet, and almost complete blockade. The proper target, after an initial attack on aircraft engine plants, either to bring over whelming pressure on her to surrender, or to reduce her capability of resisting invasion, was the basic economic and social fabric of the country. Disruption of her railroad and transportation system by daylight attacks, coupled with destruction of her cities by night and bad weather attacks, would have applied maximum pressure in support of either aim. This point of view was finally adopted. Although urban area attacks were initiated in force in March 1945, the railroad attack was just getting under way when the war ended. The total tonnage of bombs dropped by Allied planes in the Pacific war was 656400 Of this, 160800 tons, or 24 percent, were dropped on the home islands of Japan.

– Navy aircraft accounted for 6800 tons
– Army aircraft other than B-29s for 7000 tons
– B-29s for 147000 tons

By contrast, the total bomb tonnage in the European theater was 2700000 tons of which 1360000 tons were dropped within Germany’s own borders. Approximately 800 tons of bombs were dropped by China-based B-29s on Japanese home island targets from June 1944 to January 1945. These raids were of insufficient weight and accuracy to produce significant results. By the end of November 1944, 4 months after seizure of the islands, the first of the long-range bomber bases in the Marianas became operational. The number of planes originally available was small and opposition was significant. Losses on combat missions averaged 3.6 percent. The tonnage dropped prior to 9 March 1945 aggregated only 7180 tons although increasing month by month. The planes bombed from approximately 30000 feet and the percentage of bombs dropped which hit the target areas averaged less than 10 percent. Nevertheless, the effects of even the relatively small tonnage hitting the selected targets were substantial. During this period, attacks were directed almost exclusively against aircraft, primarily aircraft engine, targets. The principal aircraft engine plants were hit sufficiently heavily and persistently to convince the Japanese that these plants would inevitably be totally destroyed. The Japanese were thereby forced into a wholesale and hasty dispersal program.

The continuing pressure of immediate military requirements for more and more planes during the campaigns in the Pacific had prevented any earlier moves to disperse. When dispersal could no longer be avoided, the necessary underground tunnels, dispersed buildings, accessory facilities such as roads, railroad spurs, and power connections were not ready. As a result the decline in aircraft engine production, which shortages in special steels requiring cobalt, nickel and chrome had initiated in mid-1944, became precipitous. On 9 March 1945, a basic revision in the method of B-29 attack was instituted. It was decided to bomb the four principal Japanese cities at night from altitudes averaging 7000 feet. Japanese weakness in night fighters and antiaircraft made this program feasible. Incendiaries were used instead of high-explosive bombs and the lower altitude permitted a substantial increase in bomb load per plane. 1667 tons of bombs were dropped on Tokyo in the first attack. The chosen areas were saturated. Fifteen square miles of Tokyo’s most densely populated area were burned to the ground. The weight and intensity of this attack caught the Japanese by surprise. No subsequent urban area attack was equally destructive.

Two days later, an attack of similar magnitude on Nagoya destroyed 2 square miles. In a period of 10 days starting 9 March, a total of 1595 sorties delivered 9373 tons of bombs against Tokyo, Nagoya, Osaka, Kobe, destroying 31 square miles of those cities at a cost of 22 airplanes. The generally destructive effect of incendiary attacks against Japanese cities had been demonstrated. Thereafter, urban area attacks alternated with visual and radar attacks against selected industrial or military targets. In April, an extensive program of sowing minefields in channels and harbors at night was added. In the aggregate, 104000 tons of bombs were directed at 66 urban areas, 14150 tons were directed at aircraft factories, 10600 tons at oil refineries, 4708 at arsenals, 3500 tons at miscellaneous industrial targets, 8115 tons at air fields and seaplane bases in support of the Okinawa operation and 12054 mines were sown.

Bombing altitudes after 9 March 1945 were lower, in both day and night attacks. Japanese opposition was not effective even at the lower altitudes, and the percentage of losses to enemy action declined as the number of attacking planes increased. Bomb loads increased and operating losses declined in part due to less strain on engines at lower altitudes. Bombing accuracy increased substantially, and averaged 35 to 40 percent within 1000 feet of the aiming point in daylight attacks from 20000 feet or lower. Monthly tonnage dropped increased from 13800 tons in March to 42700 tons in July, and, with the activation of the Eighth Air Force on Okinawa, would have continued to increase thereafter to a planned figure of 115000 tons per month, had the war not come to an end. Three-quarters of the 6740 tons of bombs dropped by carrier planes on the Japanese home islands were directed against airfields, warships, >miscellaneous military targets, and one-quarter against, merchant shipping and other economic targets. Most of the warships sunk in home ports had already been immobilized for lack of fuel. The accuracy of low-level carrier plane attack was high, being at least 50 percent hits within 250 feet of the aiming point. The attack against the Hakodate – Aomori rail ferries in July 1945 sank or damaged all twelve of the ferries, 17 steel ships, and 149 smaller ships.

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Economic Effects of Air Attack Against the Japanese Home Islands

The physical destruction resulting from the air attack on Japan approximates that suffered by Germany, even though the tonnage of bombs dropped was far smaller. The attack was more concentrated in time, and the target areas were smaller and more vulnerable. Not only were the Japanese defenses overwhelmed, but Japan’s will and capacity for reconstruction, dispersal, and passive defense were less than Germany’s. In the aggregate some 40 percent of the built-up area of the 66 cities attacked was destroyed. Approximately 30 percent of the entire urban population of Japan lost their homes and many of their possessions. The physical destruction of industrial plants subjected to high-explosive attacks was similarly impressive. The larger bomb loads of the B-29s permitted higher densities of bombs per acre in the plant area, and on the average somewhat heavier bombs were used. The destruction was generally more complete than in Germany. Plants specifically attacked with high-explosive bombs were, however, limited in number. The railroad system had not yet been subjected to substantial attack and remained in reasonably good operating condition at the time of surrender. Little damage was suffered which interfered with main line operations. Trains were running through Hiroshima 48 hours after the dropping of the atomic bomb on that city. Damage to local transport facilities, however, seriously disrupted the movement of supplies within and between cities, thereby hindering production, repair work and dispersal operations.

Japan’s electric power system was properly rejected for specific attack because of the large number of small targets presented. Urban incendiary attacks destroyed the electric distribution systems in the burned-out areas simultaneously with the consumer load previously served by them. The hydro-electric generating plants and the transmission networks survived without substantial damage. Twenty-six urban steam-generating plants were damaged as an incident to other attacks, the aggregate loss of capacity being less than one-seventh of Japan’s total generating capacity. The urban area incendiary attacks eliminated completely the residential and smaller commercial and industrial structures in the affected areas and a significant number of important plants, but a portion of the more substantially constructed office buildings and factories in those areas and the underground utilities survived. By 1944 the Japanese had almost eliminated home industry in their war economy. They still relied, however, on plants employing less than 250 workers for subcontracted parts and equipment. Many of these smaller plants were concentrated in Tokyo and accounted for 50 percent of the total industrial output of the city. Such plants suffered severe damage in urban incendiary attacks.

Four hundred and seventy thousand barrels of oil and oil products, 221000 tons of food stuffs and 2 billion square yards of textiles were destroyed by air attacks. Ninety-seven percent of Japan’s stocks of guns, shells, explosives, and other military supplies were thoroughly protected in dispersed or underground storage depots, and were not vulnerable to air attack. Physical damage to plant installations by either area or precision attacks, plus decreases due to dispersal forced by the threat of further physical damage, reduced physical productive capacity by roughly the following percentages of pre-attack plant capacity :

– oil refineries, 83 percent
– aircraft engine plants, 75 percent
– air-frame plants, 60 percent
– electronics and communication equipment plants, 70 percent
– army ordnance plants, 30 percent
– naval ordnance plants, 28 percent
– merchant and naval shipyards, 15 percent
– light metals, 35 percent
– ingot steel, 15 percent
– chemicals, 10 percent

The economic consequences of the physical damage wrought by air attack are closely interrelated with the concurrent effects of the interdiction of imports, the cumulative effects of under-maintenance of plants, and the declining health, vigor and determination of the Japanese people. Let us first consider the level of Japanese industrial activity in July 1945, the last full month before surrender. Electric power and coal consumption were both almost exactly 50 percent of the peak reached in 1944. Production efficiency had, however, declined and the overall industrial output was approximately 40 percent of the 1944 peak. Output varied considerably as between industries, hit and un-hit plants, and by areas. Output of air frame was 40 percent of the 1944 peak, aircraft engines, 25 percent, ship building, 25 percent, army ordnance, 45 percent and naval ordnance, 43 percent.

Oil refining had declined to less than 15 percent of the 1943 output. Primary aluminum production was 9 percent of the 1944 peak. Although nitric acid production had declined to about 17 percent of the 1944 peak, explosives production was about 45 percent of the 1944 figure. In each one of these industries, the occasion for the decline appears to have been different. Electric power consumption fell, not because more power was not available, but because demand had declined. Coal supply was primarily limited by the decline in inter-island shipping from Hokkaido and Kyushu, and the inability of the railroad system completely to fill the gap. Despite a decline in demand, shortages of coal were universal throughout the economy airframe production was limited primarily by the continuing effects of the dispersal program brought on by the initial bombing, and aggravated by the subsequent destruction of numerous plants prior to completion of dispersal. Had the level of production been any higher, however, aluminum stocks would have been exhausted and aluminum would have become the controlling bottleneck. In any event, not enough aircraft engines were being produced to equip the airframes aircraft engine production was plagued by shortages of special steels, but in July 1945, plant damage and delay in completing the underground and dispersed plants started in the spring of the year temporarily prevented the full use of the small stocks of such steels available at the time. Output of radar and radio equipment was limited by plant capacity, the small factories supplying parts having been destroyed in the Tokyo city raids and many of the larger plants either destroyed or forced to disperse Shipbuilding and heavy ordnance production were limited by the availability of steel, Oil refineries, aluminum plants and steel plants were basically limited by lack of foreign raw materials, Explosive plants were still using up inventories of nitric acid but would shortly have had to adjust their output to the current availability of nitric acid.

The Japanese labor force had declined in efficiency due to malnutrition and fatigue, the destruction of much of the urban housing and the difficulties of local transportation. Production hours lost through all causes including absenteeism, sickness, air-raid alerts and enforced idleness rose from 20 percent in 1944 to over 40 percent in July 1945. The size of the labor force employed did not materially decline and the productive hours actually worked remained sufficiently high to indicate that such influence as manpower deficiencies may have had on the over-all level of production in July 1945, was largely as crib able to the continued drafting of highly skilled workers into the armed services and to the inefficient administration of manpower in meeting the rapidly shifting requirements resulting from bombing, rather than to overall lack of labor. A Survey investigation of production in plants employing more than 50 employees in 39 representative cities of Japan indicates that production in those plants which suffered any direct physical damage dropped off by July 1945, to 27 percent of peak output in 1944, while production in the undamaged plants fell off to 54 percent. Production in all plants in the sample, including both hit and unhit, dropped to 35 percent of peak by July 1945. It appears probable that the indirect effects of the urban raids through increased absenteeism, disruption of supply lines and administrative confusion fully compensate for diversions of manpower and material from hit to unhit plants. The difference between 54 percent, being the rate of production in un-hit plants, and 35 percent, being the average for all plants, is, therefore, a conservative indication of the impact of air attacks, both urban and precision, on production in these cities.

Even though the urban area attacks and attacks on specific industrial plants contributed a substantial percentage to the over-all decline in Japan’s economy, in many segments of that economy their effects were duplicative. Most of the oil refineries were out of oil, the alumina plants out of bauxite, the steel mills lacking in ore and coke, and the munitions plants low in steel and aluminum. Japan’s economy was in large measure being destroyed twice over, once by cutting off of imports, and secondly by air attack. A further tightening of Japan’s shipping situation, so as to eliminate remaining imports from Korea and coast wise and inter-island shipping, coupled with an attack on Japan’s extremely vulnerable railroad network, would have extended and cumulated the effects of the shipping attack already made. Much of Japan’s coastal and inter-island traffic had already been forced on to her inadequate railroads. The principal coal mines of Japan are located on Kyushu and Hokkaido. This coal traffic, formerly water borne, was moving by railroads employing the Kanmon tunnels and the Hakkodate – Aomori rail ferry. The railroads on Honshu include few main lines and these lines traverse bridges of considerable vulnerability. Japan is largely a mountainous country lacking automobile roads, trucks or the gasoline to make use of them. A successful attack on the Hakkodate rail ferry, Kanmon tunnels and 19 bridges and vulnerable sections of line so selected as to set up five separate zones of complete interdiction would have virtually eliminated further coal movements, would have immobilized the remainder of the rail system through lack of coal, and would have completed the strangulation of Japan’s economy. This strangulation would have more effectively and efficiently destroyed the economic structure of the country than individually destroying Japan’s cities and factories. It would have reduced Japan to a series of isolated communities, incapable of any sustained industrial production, incapable of moving food from the agricultural areas to the cities, and incapable of rapid large-scale movements of troops and munitions.

The Survey believes that such an attack, had it been well-planned in advance, might have been initiated by carrier-based attacks on shipping and on the Hakkodate ferry in August 1944, could have been continued by aerial mining of inland waterways beginning in December 1944, and could have been further continued by initiating the railroad attack as early as April 1945. The Survey has estimated that force requirements to effect complete interdiction of the railroad system would have been 650 B-29 visual sorties carrying 5.200 tons of high explosive bombs. Monthly tonnages equal to one and one-half times that required to effect the original interdiction should have been sufficient, in view of the Japanese lack of preparation and slowness in effecting repairs, to maintain the interdiction by destroying such bridges and other facilities as the Japanese were able to repair. The use of Azon guided bombs, which could have been made available at that time, would have greatly increased accuracy against targets of this type and reduced force requirements to approximately one-sixth of those given above. An integrated program employing both carrier planes and B-29s would have capitalized on the differing operational capabilities of each.

The economic effects of the transportation attack would have had a direct impact on the Japanese people and on their determination to continue the war. In order to bring maximum pressure on the civilian population and to complicate further the Japanese economic problems, night and bad weather attacks on urban areas could have been carried out simultaneously with the transportation attack. One of the important factors inducing Japan’s leaders to accept unconditional surrender was a realization that the Japanese armed forces had lost their ability to protect the people and that under the impact of direct air attack and lowered livelihood their confidence in victory and determination to continue the war were rapidly declining.

The Health and Morale of the Japanese Civilian Population Under Assault

Total civilian casualties in Japan, as a result of 9 months of air attack, including those from the atomic bombs, were approximately 806000. Of these, approximately 330000 were fatalities. These casualties probably exceeded Japan’s combat casualties which the Japanese estimate as having totaled approximately 780000 during the entire war. The principal cause of civilian death or injury was burns. Of the total casualties approximately 185.000 were suffered in the initial attack on Tokyo of 9 March 1945. Casualties in many extremely destructive attacks were comparatively low. Yokohama, a city of 900.000 population, was 47 percent destroyed in a single attack lasting less than an hour. The fatalities suffered were less than 5000. The Japanese had constructed extensive firebreaks by tearing down all houses along selected streets or natural barriers. The total number of buildings torn down in this program, as reported by the Japanese, amounted to 615000 as against 2510000 destroyed by the air attacks themselves. These firebreaks did not effectively stop the spread of fire, as incendiaries were dropped on both sides of the breaks. They did, however, constitute avenues of escape for the civilian population.

The Japanese instituted a civilian-defense organization prior to the war. It was not until the summer of 1944, however, that effective steps were taken to reduce the vulnerability of Japan’s civilian population to air attacks. By that time, the shortage of steel, concrete and other construction materials was such that adequate air-raid shelters could no longer be built. Each family was given the obligation of providing itself with some kind of an excavation covered with bamboo and a little dirt. In addition, tunnels were dug into the sides of hills wherever the topography permitted. Japanese planning and the means for carrying out the plans were thus deficient for a first-class civilian defense program. In spite of these limitations, such civilian defense measures as they were able to put through contributed substantially in minimizing casualties. School children and other non essential urban dwellers were evacuated to the country. Those who remained were organized to combat fires and to provide mutual assistance. The air raid warning system was generally efficient. The weight of the individual attacks was, however, far heavier than the Japanese had envisaged or were able to cope with. In the major fire attacks, the civilian defense organizations were simply over whelmed.

The growing food shortage was the principal factor affecting the health and vigor of the Japanese people. Prior to Pearl Harbor the average per capita caloric intake of the Japanese people was about 2000 calories as against 3400 in the United States. The acreage of arable land in Japan is only 3 percent of that of the United States to support a population over half as large. In order to provide the prewar diet, this arable acreage was more intensively cultivated, using more man power and larger quantities of fertilizer, than in any other country in the world; fishing was developed into a major industry; and rice, soybeans and other food stuffs amounting to 19 percent of the caloric intake were imported. Despite the rationing of food beginning in April 1941, the food situation became critical. As the war progressed, imports became more and more difficult, the waters available to the fishing fleet and the ships and fuel oil for its use became increasingly restricted. Domestic food production itself was affected by the drafting of the younger males and by an increasing shortage of fertilizers.

By 1944, the average per capita caloric intake had declined to approximately 1900 calories. By the summer of 1945 it was about 1680 calories per capita. Coal miners and heavy industrial workers received higher-than-average rations, the remaining populace, less. The average diet suffered even more drastically from reductions in fats, vitamins and minerals required for balance and adversely affected rates of recovery and mortality from disease and bomb injuries. Under nourishment produced a major increase in the incidence of beriberi and tuberculosis. It also had an important effect on the efficiency and morale of the people, and contributed to absenteeism among workers.

Survey interrogation of a scientifically designed cross-section sample of the Japanese civilian population revealed a high degree of uniformity as between city and rural sectors of the population and as between various economic and social strata in their psychological reaction to the war. A uniformly high percentage considered Japan’s greatest weaknesses to have been in the material realm, either lack of resources, productive plant or modern weapons, and her greatest strength to have been in the Yamato spirit of the Japanese people, their willingness to make every personal sacrifice, including that of life itself, for the Emperor or Japan.

The Japanese people reacted to news of the attack against the United States and its Allies with mingled feelings of fear, insecurity and hope. To a people wearied by 10 years of war in China, it was clear that this would be a major war and not an incident. The early Japanese military successes, particularly the capture of Singapore and the southern regions, were followed by a wave of optimism and high confidence. Subsequent defeats were studiously with held from the people or disguised as strategic withdrawals. Prior to the loss of Saipan confidence in eventual victory remained high in spite of exhausting work, poor nutrition and rising black market prices. In June 1944 approximately two percent of the population believed that Japan faced the probability of defeat. The fall of Saipan could not be kept from the Japanese people. Even though the psychological effect of this disaster was far greater on the Japanese leaders and intellectuals than on the mass of the population, all induces of Japanese morale began thereafter to decline. By December 1944 air attacks from the Marianas against the home islands had begun, defeats in the Philippines had been suffered, and the food situation had deteriorated; 10 percent of the people believed Japan could not achieve victory. By March 1945, when the night incendiary attacks began and the food ration was reduced, this percentage had risen to 19 percent. In June it was 46 percent, and just prior to surrender, 68 percent. Of those who had come to this belief over one-half attributed the principal cause to air attacks, other than the atomic bombing attacks, and one-third to military defeats.

Sixty-four percent of the population stated that they had reached a point prior to surrender where they felt personally unable to go on with the war. Of these, less than one-tenth attributed the cause to military defeats, one-quarter attributed the cause to shortages of food and civilian supplies, the largest part to air attack. A striking aspect of the air attack was the pervasiveness with which its impact on morale blanketed Japan. Roughly one-quarter of all people in cities fled or were evacuated, and these evacuees, who themselves were of singularly low morale, helped spread discouragement and disaffection for the war throughout the islands. This mass migration from the cities included an estimated 8.500.000 persons. Throughout the Japanese islands, whose people had always thought themselves remote from attack, United States planes crisscrossed the skies with no effective Japanese air or antiaircraft opposition. That this was an indication of impending defeat became as obvious to the rural as to the urban population.

Progressively lowered morale was characterized by loss of faith in both military and civilian leaders, loss of confidence in Japan’s military might and increasing distrust of government news releases and propaganda. People became short-tempered and more outspoken in their criticism of the government, the war and affairs in general. Until the end, however, national traditions of obedience and conformity, reinforced by the police organization, remained effective in controlling the behavior of the population. The Emperor largely escaped the criticism which was directed at other leaders, and retained the people’s faith in him. It is probable that most Japanese would have passively faced death in a continuation of the hopeless struggle, had the Emperor so ordered. When the Emperor announced the unconditional surrender the first reaction of the people was one of regret and surprise, followed shortly by relief.
The interrelation of military, economic and morale factors was complex. To a certain extent each reacted on the other. In the final analysis the Japanese military machine had lost its purpose when it could no longer protect the Japanese people from destruction by air attack.

General Takashima, when asked by the Survey as to his reaction to the Imperial Prescript, stated that surrender had become unavoidable; the Army, even should it repel invasion, could no longer protect the Japanese people from extermination.

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The Effects of the Atomic Bombs

On 6 August and 9 August 1945, the first two atomic bombs to be used for military purposes were dropped on Hiroshima and Nagasaki respectively. One hundred thousand people were killed, 6 square miles or over 50 percent of the built-up areas of the two cities were destroyed. The first and crucial question about the atomic bomb thus was answered practically and conclusively; atomic energy had been mastered for military purposes and the over whelming scale of its possibilities had been demonstrated. A detailed examination of the physical, economic, and morale effects of the atomic bombs occupied the attention of a major portion of the Survey’s staff in Japan in order to arrive at a more precise definition of the present capabilities and limitations of this radically new weapon of destruction. Eyewitness accounts of the explosion all describe similar pictures. The bombs exploded with a tremendous flash of blue white light, like a giant magnesium flare. The flash was of short duration and accompanied by intense glare and heat. It was followed by a tremendous pressure wave and the rumbling sound of the explosion. This sound is not clearly recollected by those who survived near the center of the explosion, although it was clearly heard by others as much as 15 miles away. A huge snow-white cloud shot rapidly into the sky and the scene on the ground was obscured first by a bluish haze and then by a purple-brown cloud of dust and smoke. Such eyewitness accounts reveal the sequence of events . At the time of the explosion, energy was given off in the forms of light, heat, radiation, pressure.

– 1 – The complete band of radiations, from X-and gamma-rays, through ultraviolet and light rays to the radiant heat of infra-red rays, traveled with the speed of light;
– 2 – The shock wave created by the enormous pressures built up almost instantaneously at the point of explosion but moved out more slowly, that is at about the speed of sound;
– 3 – The super heated gases constituting the original fire ball expanded outward and upward at a slower rate;
– 4 – The light and radiant heat rays accompanying the flash traveled in a straight line and any opaque object, even a single leaf of a vine, shielded objects lying behind it;
– 5 – The duration of the flash was only a fraction of a second, but it was sufficiently intense to cause third degree burns to exposed human skin up to a distance of a mile;
– 6 – Clothing ignited, though it could be quickly beaten out, telephone poles charred, that chroofed houses caught fire;
– 7 – Black or other dark-colored surfaces of combustible material absorbed the heat and immediately charred or burst into flames;
– 8 – White or light-colored surfaces reflected a substantial portion of the rays and were not consumed;
– 9 – The heavy black clay tiles which are an almost universal feature of the roofs of Japanese houses bubbled at distances up to a mile. Test of samples of this tile by the National Bureau of Standards in Washington indicates that temperatures in excess of 1800° C. must have been generated in the surface of the tile to produce such an effect;
– 10 – The surfaces of granite blocks exposed to the flash scarred and stalled at distances up to almost a mile;
– 11 – In the immediate area of ground zero the point on the ground immediately below the explosion, the heat charred corpses beyond recognition;
– 12 – Penetrating rays such as gamma-rays exposed X-ray films stored in the basement of a concrete hospital almost a mile from ground zero;
– 13 – Symptoms of their effect on human beings close to the center of the explosion, who survived other effects thereof,were generally delayed for two or three days;
– 14 – The bone marrow and as a result the process of blood formation were affected. The white corpuscle count went down and the human processes of resisting infection were destroyed. Death generally followed shortly thereafter;
– 15 – The majority of radiation cases who were at greater distances did not show severe symptoms until 1 to 4 weeks after the explosion. The first symptoms were loss of appetite, lassitude and general discomfort. Within in 12 to 48 hours, fever became evident in many cases, going as high as 104° to 105° F., which in fatal cases continued until death. If the fever subsided, the patient usually showed a rapid disappearance of other symptoms and soon regained his feeling of good health;
– 16 – Other symptoms were loss of white blood corpuscles, loss of hair, and decrease in sperm count. Even though rays of this nature have great powers of penetration, intervening substances filter out portions of them. As the weight of the intervening material increases the percentage of the rays penetrating goes down. It appears that a few feet of concrete, or a somewhat greater thickness of earth, furnished sufficient protection to humans, even those close to ground zero, to prevent serious after effects from radiation.

The blast wave which followed the flash was of sufficient force to press in the roofs of reinforced-concrete structures and to flatten completely all less sturdy structures. Due to the height of the explosion, the peak pressure of the wave at ground zero was no higher than that produced by a near-miss of a high-explosive bomb, and decreased at greater distances from ground zero. Reflection and shielding by intervening hills and structures produced some unevenness in the pattern. The blast wave, however, was of far greater extent and duration than that of a high-explosive bomb and most reinforced-concrete structures suffered structural damage or collapse up to 700 feet at Hiroshima and 2000 feet at Nagasaki. Brick buildings were flattened up to 7300 feet at Hiroshima and 8500 feet at Nagasaki. Typical Japanese houses of wood construction suffered total collapse up to approximately 7300 feet at Hiroshima and 8200 feet at Nagasaki.

Beyond these distances structures received less serious damage to roots, wall partitions, and the like. Glass windows were blown out at distances up to 5 miles. The blast wave, being of longer duration than that caused by high-explosive detonations, was accompanied by more flying debris. Window frames, doors, and partitions which would have been shaken down by a near-miss of a high-explosive bomb were hurled at high velocity through those buildings which did not collapse. Machine tools and most other production equipment in industrial plants were not directly damaged by the blast wave, but were damaged by collapsing buildings or ensuing general fire.

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hiroshima-02The above description mentions all the categories of the destructive action by the atomic-bomb explosions at Hiroshima and Nagasaki. There were no other types of action. Nothing was vaporized or disintegrated; vegetation is growing again immediately under the center of the explosions; there are no indications that radio-activity continued after the explosion to a sufficient degree to harm human beings. Let us consider, however, the effect of these various types of destructive action on the cities of Hiroshima and Nagasaki and their inhabitants. Hiroshima is built on a broad river delta; it is flat and little above sea level. The total city area is 26 square miles but only 7 square miles at the center were densely built up. The principal industries, which had been greatly expanded during the war, were located on the periphery of the city. The population of the city had been reduced from approximately 340000 to 245000 as a result of a civilian defense evacuation program. The explosion caught the city by surprise.

hiroshima-03An alert had been sounded but in view of the small number of planes the all-clear had been given. Consequently, the population had not taken shelter. The bomb exploded a little north west of the center of the built-up area. Every one who was out in the open and was exposed to the initial flash suffered serious burns where not protected by clothing. Over 4 square miles in the center of the city were flattened to the ground with the exception of some 50 reinforced concrete buildings, most of which were internally gutted and many of which suffered structural damage. Most of the people in the flattened area were crushed or pinned down by the collapsing buildings or flying debris.

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hiroshima-05Shortly thereafter, numerous fires started, a few from the direct heat of the flash, but most from overturned charcoal cooking stoves or other secondary causes. These fires grew in size, merging into a general conflagration fanned by a wind sucked into the center of the city by the rising heat. The civilian-defense organization was overwhelmed by the completeness of the destruction, and the spread of fire was halted more by the air rushing toward the center of the conflagration than by efforts of the fire-fighting organization. Approximately 60 to 70000 people were killed, and 50.000 were injured. Of approximately 90000 buildings in the city, 65.000 were rendered unusable and almost all the remainder received at least light superficial damage. The underground utilities of the city were undamaged except where they crossed bridges over the rivers cutting through the city. All of the small factories in the center of the city were destroyed. However, the big plants on the periphery of the city were almost completely undamaged and 94 percent of their workers unhurt.

hiroshima-09These factories accounted for 74 percent of the industrial production of the city. It is estimated that they could have resumed substantially normal production within 30 days of the bombing, had the war continued. The railroads running through the city were repaired for the resumption of through traffic on 8 August, 2 days after the attack. Nagasaki was a highly congested city built around the harbor and up into the ravines an driver valleys of the surrounding hills. Spurs of these hills coming down close to the head of the bay divide the city roughly into two basins. The built-up area was 3.4s square miles of which 0.6 square miles was given over to industry. The peak wartime population of 285.000 had been reduced to around 230.000, by August 1945, largely by preraid evacuations. Nagasaki had been attacked sporadically prior to 9 August by an aggregate of 136 planes which dropped 270 tons of high explosives and 53 tons of incendiary bombs.

Some 2 percent of the residential buildings had been destroyed or badly damaged; three of the large industrial plants had received scattered damage.The city was thus comparatively intact at the time of the atomic bombing. The alarm was improperly given and therefore few persons were in shelters. The bomb exploded over the northwest portion of the city; the intervening hills protected a major portion of the city lying in the adjoining valley. The heat radiation and blast actions of the Nagasaki bomb were more intense than those of the bomb dropped over Hiroshima. Reinforced-concrete structures were structurally damaged at greater distances; the heavy steel-frame industrial buildings of the Mitsubishi steel works and the arms plant were pushed at crazy angles away from the center of the explosion. Contrary to the situation at Hiroshima, the majority of the fires that started immediately after the explosion resulted from direct ignition by the flash. Approximately 40000 persons were killed or missing and a like number injured. Of the 52000 residential buildings in Nagasaki 14000 were totally destroyed and a further 5.400 badly damaged.

hiroshima-06Ninety-six percent of the industrial output of Nagasaki was concentrated in the large plants of the Mitsubishi Co. which completely dominated the town. The arms plant and the steel works were located within the area of primary damage. This estimated that 58 percent of the yen value of the arms plant and 78 percent of the value of the steel works were destroyed. The main plant of the Mitsubishi electric works was on the periphery of the area of greatest destruction. Approximately 25 percent of its value was destroyed. The dockyard, the largest industrial establishment in Nagasaki and one of the three plants previously damaged by high-explosive bombs, was located down the bay from the explosion. It suffered virtually no new damage. The Mitsubishi plants were all operating, prior to the attack, at a fraction of their capacity because of a shortage of raw materials. Had the war continued, and had the raw material situation been such as to warrant their restoration, it is estimated that the dockyard could have been in a position to produce at 80 percent of its full capacity within 3 to 4 months; that the steel works would have required a year to get into substantial production; that the electric works could have resumed some production within 2 months and been back at capacity within 6 months; and that restoration of the arms plant to 60 to 70 percent of former capacity would have required 15 months. Some 400 persons were in the tunnel shelters in Nagasaki at the time of the explosion. The shelters consisted of rough tunnels dug horizontally into the sides of hills with crude, earth-filled blast walls protecting the entrances. The blast walls were blown in but all the occupants back from the entrances survived, even in those tunnels almost directly under the explosion. Those not in a direct line with the entrance were uninjured. The tunnels had a capacity of roughly 100.000 persons. Had the proper alarm been sounded, and these tunnel shelters been filled to capacity, the loss of life in Nagasaki would have been substantially lower.

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The Survey has estimated that the damage and casualties caused at Hiroshima by the one atomic bomb dropped from a single plane would have required 220 B-29s carrying 1200 tons of incendiary bombs, 400 tons of high-explosive bombs, 500 tons of antipersonnel fragmentation bombs, if conventional weapons, rather than an atomic bomb, had been used. One hundred and twenty-five B-29s carrying 1200 tons of bombs would have been required to approximate the damage and casualties at Nagasaki. This estimate pre-supposed bombing under conditions similar to those existing when the atomic bombs were dropped and bombing accuracy equal to the average attained by the Twentieth Air Force during the last 3 months of the war.



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