Hiroshima Under Atomic Bomb Attack

8:15 a.m., August 6, 1945

Weather fine and cloudless.

Three United States B-29's had approached the City of Hiroshima from the northeastern direction, maintaining an altitude of approximately 8,500 meters, as observed by an anti-air artillery unit. One of them then stopped its engines and as it glided over across the central part of the city, it dropped a single Atomic Bomb, and making an abrupt right-angled turn, darted away at full speed. At the time of the bomb explosion, it had flown 16 kilometers to the northwest, somewhere in the San-in District.

The bomb fell rapidly with a trail of thick, red column of fame in its wake and one and a half minutes later, at the height of about 570 meters above the ground level, it exploded with a terrific detonation in a fire-ball 60 meters in diameter. The temperature of this fire-ball, often referred to as "miniature sun", is estimated at 300,000 °C, 1/10,000 of a second after the detonation.

The terrific explosion sent reddish-blue or dark-brown flames shooting out against the ground at an astounding velocity, radioactivating some forty percent of the city area. Simultaneously a cluster of white smoke became visible, which, centering around a mass of dark-red cloud and enveloped in a yellowish mass of cloud, mushroomed upward, topped with a crest of white cloud.

This cumulus-like development is referred to as the "Atomic Cloud". The atomic Energy Commission of the United States disclosed that it rose to a height of 3,000 meters in 48 seconds and in 8½ minutes it had reached 9,000 meters, just below the stratosphere.

Fifteen minutes after the Atomic Cloud started surging upward, carrying with it radioactivated particles, rain began to fall. For the first two hours it was muddy rain and then there was drizzling for another six hours. This brought the radioactive particles back down to the ground. Pressure waves from the blast flattened practically all buildings within 2.5 kilometers of the hypocenter.

About twenty minutes after the explosion, fires broke out all over the city, and the conflagration into which they developed rapidly, devoured all the buildings it could lay its hands on. In this unprecedented holocaust, more than 260,000 persons lost their lives and more than 100,000 were more or less injured.

With the bomb were dropped three automatic signaling units contained in cylinders 80 cm long and 15 cm in diameter, equipped with parachutes. They were later found in the woods of Kameyama, Asa-gun. Each consisted of four compartments which housed pressure-measuring apparatus, vacuum tubes and batteries, with antenna several meters long attached to the upper end.

Further investigations showed that they were transmitters operating on 20-meter bands, with an output power of 40 watts, and so designed that the wave length changed with the atmospheric pressure under which it operated, thus enabling the operators to ascertain whether the bomb actually detonated or not.

Determination of the Explosion Center

The surface of rocks and stones that received direct heat ray was either melted or exfoliated, and the sharp contrast formed by the unaffected surface enabled measurement of the angles of elevation and azithmus of the explosion point at each point of observation. Such observations were carried out at 6,542 different locations. The actual point of explosion was then determined by drawing lines on a map through each point of observation at the angles measured and marking the point of convergence. If we call the explosion point in the air "epicenter", and its vertical projection on the ground "hypocenter", the hypocenter was found to be in the yard of the Shima Hospital, Saiku-machi, and the epicenter about 570 meters above this point. Both the hypocenter and the epicenter had a diameter of about 60 meters.

Measurement of the Epicenter & Fireball

The part of granite surface which was exposed to the direct beam of the heat rays for a brief duration of 1/10,000 second, was either melted or exfoliated, resulting in a sharp contrast with that part which was not affected by the rays being intercepted by objects standing in the way. 6,542 locations were carefully selected where such lines of demarcation were clear and distinct and the directional angle and the elevation were measured. Some of the results are tabulated in the following:

Obiect & Location	Angle of Elevation (degree)	Altitude (meter)	Distance from Hypocenter (meter)
Tombstone, Myorenji Temple, Fukuro-machi	54	550. 552	400
"	55	571. 260	400
Tombstone, Kokutaiji Temple, Komachi	54	578. 0796	420
Tombstone, Myohoji Temple, Zaimoku-cho	75	585. 1406	380
Tombstone, Myohoji Temple, Zaimoku-cho/td>	58	608. 1258	380
Tombstone, Jisenji Temple, Nakajima Honmachi	65	557. 5726	260
Tombstone, Denpukuji Temple, Zaimoku-cho	56	548. 5072	370
Tombstone, Senshoji Temple, Shioya-cho	74	592. 8597	170
Tombstone, Jokokuji Temple, Nishijikata-machi	42	558. 2480	320
Tombstone, Jokokuji Temple, Nishi Jikata-machi	44	598. 7278	620
Tombstone, Keizoin Temple, Zaimokucho	60	588. 8970	340
"	58	544. 1122	340
Tombstone, Joenji Temple, Zaimoku-cho	60	606. 2175	350
"	58	560. 2175	350
Tombstone, Denpukuii Temple, Zaimoku-cho	58	592. 1221	370
Tombstone, Shojunji Temple, Togiya-cho	57	523.5558	340
Granite block, Gokoku Shrine, Motomachi	72	523. 2056	170
Bank of Japan, Fukuro-machi	58	576. 1188	360
Yasuda Bank, Hirataya-cho	49	598. 1934	520
Honkawa Primary School, Kajiya-cho	60	606.2175	350
Shinohashi Bridge, Nishijikata-machi	46	600. 6071	580
Yorozuyo Bridge, Kako-machi	38	571. 8074	860

Rainfall Following the Explosion

Rain started fifteen minutes after the explosion and continued for about four hours, but at most places it started twenty minutes to one hour after the blast. Along the frontal line rain started one to two hours after the flash. At Hakushima it started four hours later.

The first rain started in the area near the explosion point, and then spread out to the north-western sector of the city, finally extending to the hilly regions to the west of the city. On the map the area of rainfall shows an elongated oval, with its extreme end reaching out as far as Yamagata Gun.

Such rainfall is believed to have been caused by the ascending air current stirred up by the explosion, augmented by a similar current developed by the fires that followed and by the frontal atmospheric pressure.

Fires

Simultaneously with the explosion, the heat rays kindled fire at various places all over the city, even as far as 3,700 meters from the explosion center. The wood and paper sliding doors, straw, electric poles, wooden houses, and clothes, particularly of darker colors, were some of the things that easily caught fire from the heat rays and thus constituted major sources of the subsequent conflagration. Remarkable instances of such ignition are seen in electric poles, wooden houses, railroad sleepers, cargo mine post and bamboo bushes, all of which show charred surfaces facing the explosion center.

Kitchen fires and other sources of heat in the flattened houses were further causes of the conflagration. Thus, five minutes after the explosion, smoke began to go up and twenty minutes later, fires were observed in Komachi, Kokutaiji, Nakajima-cho, Tokaichi-machi, Temma-cho and Funairi-cho area, rapidly spreading over the entire city.

Those who barely survived the terrific blast had received ugly burns from the rays and suffered from wounds caused by falling timbers and fragments of glass. In daze and confusion, they did not even know what to do to save their own lives. With

all means of communication completely paralyzed, rescue parties from neighboring communities were delayed, and with all fire-fighting organs destroyed, the whole city was left to the mercy of raging fires, so that by 2 o'clock in the afternoon, the entire city was enveloped in a vast sea of flames. Although its intensity gradually decreased toward evening, the fire continued into night, with the flames licking at the night sky. Toward 10 o'clock the next morning it gradually became localized, but still kept on burning for three more days, and the embers went on smoldering for over a week.

How Building Stones were Affected by the Heat Rays

A. Granite

Granite had been in extensive use as building stone and tombstone in Hiroshima, as it lies in the center of granite producing

areas of the Chugoku District. Most of the granite existing in Hiroshima had come from quarries in Kurahashi-Jima, Hiroshima

Prefecture; in the vicinities of Yanai Machi, Yamaguchi Prefecture; in Mannari, Okayama City, and in the islands near the Shikoku District.

Heat rays that hit the granite surface caused remarkable changes, creating a new rough surface. Within 900 meters from the explosion point, such surface consisted of new minerals formed by the melting of darker substances such as amphibole, augite 'and biotite, while quartz and feldspar were peeled off, presumably by the rapid intermittent expansion that took place. In some cases they were also melted, and in still others, heat rays reflected by surrounding objects caused them to peel off.

The surface thus affected is sharply contrasted with the surface that was left intact and remains, as it were, a permanent shadow. The line of demarcation, indicating the direction of the rays, was followed to determine the exact location of the explosion point.

The fires that developed after the blast also affected granite, but in this case, the surface invariably exfoliated like onion peels.

B. Andesite

Instances of andesite affected by the heat rays are much rarer, as it had not been in common use as building stone in this locality. One instance, however, was found near the hypocenter, on a stone wall that enclosed the Sei Hospital, Saiku-machi.

There the angite and andesite showed formation of a thin black vitreous layer on the surface, of tiny granules, while all the other tiny crystals had melted. The glassy substance on the thin layer is of black color, tinged with brown.

The andesite slabs paved around what was the main building of the Gokoku Shrine also show vitreous surface due to having been exposed to a terrific temperature far above its melting point, and it is interesting to note that pebbles which happened to be upon these slabs have left their "shadows" clearly imprinted on the exposed surfaces.

Augite and andesite slabs in the garden of Sanyo Museum, Fukuromachi, 450 meters from the hypocenter, show spotwise peeling of the surface. There are some instance in which vitreous surfaces were formed by melting. Andesite rocks near Kami Nagarekawa about 1,000 meters from the hypocenter, invariably show spotwise peelins.

C. Sedimentary and Metamorphic Rocks

These are extremely rare in the city area. In the cemetery of Sairenji Temple, about 50 meters from the hypocenter, we find the surface of biotite gneiss melted to a remarkable degree into spots which are seen protruding. Near Gokoku Shrine, 350 meters from the hypocenter, exfoliation of the surface prevailed with little melting. There the surface of chlorite schists shows formation of black vitreous film. Similar formation resulting from the melting is observed on the surface of chlorite schists located in the cemetery of Kokutaiji Temple, Komachi.

Effects on the Rooftiles

Most of the rooftiles in the city were of black color. At the hypocenter and near it, rooftiles were found whose surface had melted under the heat rays. The vitreous film thus formed was either of olive or brown color. The size of the glassy bubbles contained in the film and the thickness of the film increased with the proximity of the location to the hypocenter.

Observations by unaided eye seem to indicate that the surface melting of roof tiles took place to as far out as 600 meters from the hypocenter. It may be noted that roof tiles are generally of identical composition, most of them coming from the Iyo region of the Shikoku District where in terms of petrology they are manufactured from clay decomposed from quartz porphyry and granite.

Shadows left on Tar-Paved Bridge

Numerous shadows could be observed imprinted on building materials, steel plates and ceramic wares. Most striking are those seen on the Yorozuyo Bridge near what was the Prefectural Office. Ten persons walking east to west have left their shadows permanently imprinted on the railing and on the tar-paved surface. It points to an intense beam of rays flashing for an extremely short duration. Another marked instance is the shadow of a staircase clearly visible on the outside wall of a gas tank in Minamimachi, 2,000 meters from the hypocenter.

Effects on Human Body

Three factors are supposed to have worked together almost simultaneously with the explosion, on human bodies: first, the terrific heat, which, within 4,000 meters from the hypocenter, burnt the exposed skin, which was then ripped by the following blast. The skin thus affected either peeled off or hung in peels. A greater majority of the persons with more than 20% of the body surface thus wounded either died on the spot or a short period later. Second, the violent concussion, by which many persons were pinned down or buried under falling structures, or showered in the splinters of flying glass. Third, the radioactivity: wounds caused by it are further classified into primary and secondary radiation wounds.

The Medical Group of the Tokyo Imperial University summarized the development of radiation effects on human body in the following table:

Stage	Duration	Symptom
1st Stage	Two weeks from the explosion.	Acute
2nd Stage	Six weeks, starting with the third week and continuing through the fourth.	Quasi-acute, followed by complications.
3rd Stage	Eight weeks, starting with the third month and continuing through the fourth.	Recuperative
4th Stage	After the fifth month.	Residuary

Casualties:

More than 240,000 persons are believed to have been killed, of which the citizens, students, patriotic corps and others numbered about 170,000 and military personnel aggregated to about 70,000. 51,012 persons are known to have been seriously injured and 105,543 slightly injured. 6,738 persons have been reported missing

Death rates at different distance from the hypocenter have been checked by the medical Group of the Tokyo Imperial University and their findings are tabulated as follows:

Distance from Hypocenter in Kilometers	Percentage of Aggregate Deaths	Percentage of Deaths on Aug. 6
0 - 0.5	98.4	90.4
0.6 - 1.0	90.0	59.4
1.1 - 1.5	45.5	19.6
1.6 - 2.0	22.6	11.1
0 - 2.0	56.0

Survivors in Hiroshima Prefecture and Hiroshima City (As of October 1, 1951)

	Male	Female	Total
Hiroshima City	42,591	55,511	98,102
Hiroshima Prefecture (Excluding Hiroshima City)	14,641	12,424	27,065
Total	57,232	67,935	125,167

Distribution of Survivors

	Male	Female
Hokkaido	413	168
Aomori	84	19
Iwate	117	28
Akita	112	19
Miyagi	209	47
Yamagata	142	21
Fukushima	210	54
Ibaragi	293	67
Tochigi	211	36
Gunma	216	26
Chiba	490	121
Saitama	297	75
Tokyo	1,842	1,094
Kanagawa	627	298
Niigata	2,270	574
Toyama	66	31
Ishikawa	117	53
Fukui	149	41
Nagano	235	29
Gifu	271	60
Yamanashi	131	21
Shizuoka	354	84
Aichi	734	206
Miye	331	100
Shiga	262	98
Kyoto	532	289
Nara	172	68
Osaka	1,485	877
Wakayama	317	69
Hyogo	1,28	745
Tottori	482	104
Shimane	1,098	569
Okayama	1,311	529
Yamaguchi	2	1,846
Kagawa	455	247
Tokushima	273	104
Ehime	849	561
Kochi	289	153
Fukuoka	1,077	777
Oita	412	255
Saga	191	113
Nagasaki	597	434
Kumamoto	326	172
Miyazaki	242	185
Kagoshima	391	184
Total	81,593	79,586

Physical Damage

Area Damaged—The city limits of Hiroshima enclose a total area of 18,000 acres, of which 8,200 acres had been in use for various city activities. Physical damage extended over a total area of more than 7,500 acres, and fires following the bomb explosion consumed everything in an area amounting to 3,280 acres.

Buildings and Houses—The blast alone brought total destruction to 6,820 building and houses, and dealt major damage to 3,750. The fires that followed the blast burnt down 56,111 buildings and houses, besides seriously damaging 2,290. The following table illustrates the distribution of the damage in terms of distance from the hypocenter.

Distance from Hypocenter	No. of Bldg. Prior to Bombing	No. of Bldg. Destroyed or Damaged By Blast and/or Fire	%
0-0.5 km	5,608	5,608	100
0.5-1.0	14,059	14,059	100
1.0-1.5	14,598	14,598	99.9
1.5-2.0	10,752	10,451	99.2
2.0-2.5	12,168	10,557	86.7
2.5-3.0	7,383	6,280	85.0
3.0-3.5	2,433	2,050	84.2
3.5-4.0	3,727	3,066	82.2
4.0-4.5	1,180	874	74.1
4.5-5.0	1,577	947	60.1
More that 5.0	2,686	474	17.6

Roads and Bridges—Of the city-maintained road that ran a total length of 454,872 meters, damage was incurred by a total length of 14,600 meters, or 16%. Of the 225 bridges in the city, 78 were seriously damaged, including 22 ferro-concrete bridges, with a total span of 2,004 meters, and 56 wooden, earthen or stone bridges, with a total span of 2,050 meters.

Water Supply System—The city water supply system was almost entirely wrecked by the bomb explosion. At the upstream water source electric lines in the chlorination basin and other underground installations were torn away. Of the 17 pipe-carrying bridges, 6 were torn down and 7 seriously damaged. Water mains throughout the city were fractured and water spouted from the damaged fire hydrants at numerous points. Terminal supply equipment suffered total destruction in buildings completely burnt down, and in buildings wrecked by the blast the majority of the equipment suffered from serious leakage.

Sewerage System—Of the 13 pumping stations, 7 were completely demolished by the fire, 3 by the blast, and the remaining 3 suffered partial destruction. All of the 6 irrigation stations were reduced to ashes. Manhole plates were removed and outlet terminals damaged, so that sewage practically ceased to flow. Only the sluice gates came out with comparatively slight damage.

City Establishments—City-owned buildings and establishments incurred serious damage, which was then estimated at ¥18,217,951.

The following chart summarizes the loss.

Classification	Totally destroyed by fire	Totally destroved by blast	Partly destroyed by blast	Slightly damaged	Total
Educational Establishments	19	3	7	14	43
Health & Sanitation	6	0	2	0	8
Social Welfare Institutions	5	0	2	2	9
Industrial Plants	3	1	3	0	7
City Housing	1	3	4	0	8
Water Supply	4	0	2	0	6
Sewerage	16	4	4	8	32
City Office & Others	2	2	0	1	5
Total	56	13	24	25	118

Summary of Damage by Pressure Waves

Velocity (m/sec)	Duration (sec)	Distance (km)	Description
22	1.25	3.50	Slight damage to window frames & doors; medium damage to plastered wall; window glass blown to bits.
27	1.23	3.25	Electric poles charred; breakage of roof & wall of ferro-concrete buildings.
31	1.20	3.00	Damages to buildings become more limited and partial as distance exceeds 3 km.
36	1.15	2.75	Most houses collapse; heat rays kindle fires; heavy damage to plastered walls.
45	1.12	2.50	(Damage not more than medium as distance exceeds 2.5 km.) Dwelling units suffer major destruction; Window frames and doors ripped off.
56	1.06	2.25	Brick buildings more than two stories suffer from heavy to total destruction.
		2.00	Damages serious over whole area with ferro-concrete buildings suffering major destruction.
71	0.98	1.75	Medium fracture in brick walls 9 inches thick; street cars damaged; total collapse of brick buildings more than two stories high; fractures in brick wall 12 inches thick; total collapse of steel-framed tuildings & light concrete structures.
90	0.90	1.50
120	0.77	1.25	Total collapse of reinforced concrete smoke stack 8 inches thick & brick walls 18 inches thick.
170	0.62	1.00	Total collapse of all buildings except earthquake-proof reinforced concrete buildings.
250	0.45	0.50	(Limit of major destruction to earthquake-proof reinforced concrete buildings.)
360	0.37	0.25	Total collapse of interior and major damage to roof of ferro-concrete buildings; upheavals of steel-girdered bridges.

Schools and Libraries—Of the more than 80 schools in the city, none escaped the disaster without any loss. The details are given in the following table:

Classification	Number of institutions prior to bombing	Completely burnt down	Completely demolished by blast	Partially destroved by blast	Usable
City Primary Schools	37	14	3	7	13
State, Prefectural & Private Primary Schools	5	0	0	0	0
Middle Schools	29	14	8	0	7
Colleges	12	3	2	2	5
University	1	1	0	0	0
Libraries	2	1	0	0	1
Total	86	38	13	16	19
% to total		44%	15%	18%	22%

Important Government Offices, Business Firms & Factories

Classification	No. of establishment suffering partial or total destruction	Remarks
Government Offices	28	24 completely destroyed by fire
Major Business Firms & Factories	28	16 completely destroyed by fire
Other Business Firms & Factories	6,184	6,028 partially destroyed by fire; 156 partially destroyed by blast
Temples & Shrines	51	All completely destroyed by fire
Total	6,291

Transportation & Communication Facilities, Public Utilities

Classification	Damage
Transportation	Street cars 108, of which 22 completely burnt; 3 partially burnt, besides 83 more or less damaged. Electric poles 393, of which 302 wooden; 91 steel. 28,450 meters of street car track lines; 94,350 meters of overhead cables; 65,900 meters of power lines. Motor cars 68; office buildings 50 burnt; telephone lines 66,000 meters. Casualties 541, of which 162 killed, 287 injured and 2 missing.
Communication	Central Telephone Exchange suffered from total burning of all apparatus & equipment. West Telephone Exchange suffered major damage to apparatus & equipment. No. of telephone units destroyed 6,771. Heavy damage to telephone lines.
Gas	Gas tank put out of use by major breakage; gas supply decreased by 185,000 cubic meters.
Electricity	Electric lights 260,788 Electric poles 4,539 Electric lines 210,000 Power houses 2 (entirely disappeared)

Railway—Train service was suspended for 56 hours until one fallen bridge, 100-meter-long embankment submersion and heat-damaged rails 120 meters long were restored. Damage to carriages are given in detail in the following chart:

Classification	Totally burnt	Partly burnt	Seriously damaged	Moderately damaged	Slightly damaged	Total
Passenger coaches	5	—	16	28	36	85
Freight wagons	39	3	4	8	9	63
Electric cars	6	—	2	2	2	12
Motor cars	5	—	—	—	—	5
Total	55	3	22	38	47	165

Included in the table above is the damage incurred by Train No. 337, which was derailed on the Enko Bridge and plunged into the water. 26 of the wagons were completely razed; 2 partially destroyed by fire: 4 seriously damaged; 7 moderately damaged and 6 slightly damaged. Of the 45 wagons, only 4 remained intact.

Damages Compared Between Hiroshima & Nagasaki

1. Building Damages & Casualties

	Hiroshima	Nagasaki
Time of Bombing	8:15 am. Aug. 6 1945	11:02 am. Aug. 9, 1945
Burnt-out Area	3,280 acres	1,665 acres
Houses completely destroyed by fire	56,111 units	11,574 units
Houses completely destroyed by blast	6,820 units	1,326 units
Houses more than partially destroyed by blast	3,750 units	5,509 units
Houses partially destroyed by fire	2,290 units	73,884 units
Persons killed	260,000	74,904
Persons wounded	163,293

2. Radii of Heat-ray Effects on Rooftile & Granite

	Hiroshima	Nagasaki
Altitude of Explosion	570 meters	490 meters
Surface melting of Rooftile	Within 600 meters of the hypocenter, or within 830 meters of the epicenter.	Within 950—1,000 meters of the hypocenter, or, within 1,050—1,120 meters of the epicenter.
Granite Exfoliation	Within 1,000—1,100 meters of the hypocenter, or within 1,150—1,250 meters of epicenter.	Within 1,600 meters of the hypocenter, or within 1,680 meters of the epicenter.
Surface Melting of Granite	Within approximately 900 meters of the hypocenter.	(Unascertainable due to non-existence of granite structures.)