Duck and cover

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Official 1951 American Civil Defense film, geared towards children, portraying the act of ducking and covering by Bert The Turtle

Duck and Cover is a method of personal protection against the effects of a nuclear explosion, which the United States government taught to generations of United States school children from the early 1950s until the end of the Cold War in the late 1980s.[citation needed] It was intended to protect them in the event of both an unexpected nuclear attack, which, they were told, might come at any time without warning (although with the Ballistic Missile Early Warning System, and the Pinetree Line of the era, a successful surprise attack was becoming less likely) and in the event sufficient warning is given. Under the conditions of a surprise attack, immediately after they saw a flash they had to stop what they were doing and get on the ground under some cover—such as a table, or at least next to a wall—and assume a prone like position, lying face-down and covering their exposed skin and back of their heads with their clothes, or if no excess clothes such as a coat was available, to cover the back of their heads with their hands. Similar instructions were given in 1964 in the United Kingdom by Civil Defence Information Bulletin No. 5.[1] and, in the 1980s, by the Protect and Survive series.[2] Under the conditions where sufficient warning is given, they were told to find the nearest Civil Defense shelter, or if one could not be found, any well built building to stay and shelter in.

Proponents of duck and cover argue that thousands could be saved by following the simple advice, without which, to give one easily understand example, people, being naturally inquisitive, would instead run to windows to try to locate the source of the immensely bright flash generated at the instant of the explosion. During which time, unbeknownst to them, the slower moving blast wave would rapidly be advancing towards their position, only to arrive and cause a window glass implosion, shredding onlookers.[3]

In the Testimony of Dr. Hiroshi Sawachika for example, although he was sufficiently far away from the Hiroshima bomb himself and not behind a pane of window glass when the blast wave arrived, those in his company who were, had serious blast injury wounds, with broken glass and pieces of wood stuck into them.[4]

According to the 1946 book Hiroshima, in the days between the atomic bombings of Hiroshima and Nagasaki in Japan during World War II, one Hiroshima policeman went to Nagasaki to teach police about ducking after the atomic flash. As a result of this timely warning, not a single Nagasaki policeman died in the initial blast. This allowed more surviving Nagasaki police to organize relief efforts than in Hiroshima. Unfortunately, the general population was not warned of the heat/blast danger following an atomic flash because of the bomb’s unknown nature. Many people in Hiroshima and Nagasaki died while searching the skies for the source of the brilliant flash.

When people are indoors, running to windows to investigate the source of bright flashes in the sky still remains a common inquisitive human response to experiencing a flash, so although the advice to duck and cover is over half a century old, ballistic glass lacerations caused the majority of the 1000 human injuries following the Chelyabinsk meteor air burst of February 15, 2013.[5] This natural human behavior was also observed amongst people in the vicinity of Hiroshima and Nagasaki, and therefore a human behavior advised against in Duck and Cover advice, and in the film of the same name.[6]

The dangers of viewing explosions behind common window glass was known of before the nuclear age began, being a common source of injury and death from large chemical explosions, with one such incident being the 1944 Port Chicago explosion, where about 4,600 tons (4,173 metric tons)[7] of chemical explosive accidentally detonated in a series of events. Survivor, Robert Routh, reported enjoying the sight of the wonderfully bright fireball and then suddenly never seeing anything again, as following the blast arriving, he was permanently blinded by the window he was viewing the fireball from, shards of which pierced his eyes and face.[8]

Similar to duck and cover is the advice to "Drop, Cover and Hold On" which is taught in areas prone to earthquakes. Schools in some tornado-prone areas of the United States also practice tornado drills that involve children squatting and covering the backs of their heads.[9][10]

Background[edit]

A Duck & Cover movie poster

The United States' monopoly on nuclear weapons was broken in 1949 when the Soviet Union tested its first nuclear device(Joe-1), and many in the government and public perceived that the United States was more vulnerable than it had ever been before. Duck-and-cover exercises quickly became a part of Civil Defense drills that every American citizen, from children to the elderly, practiced to be ready in the event of nuclear war. In 1950, during the first big Civil Defense push of the Cold War; the movie Duck and Cover was produced (by the Federal Civil Defense Administration) for school showings in 1951.

This was during the height of the Korean War(1950-1953), which narrowly avoided becoming another World War, and coincided with the first Desert Rock exercises in the Nevada desert which were designed to familiarize the US military with fighting alongside battlefield nuclear weapons, as it was feared the a resolution to the Korean War might need the theatre of operations to first expand across the border into the Peoples Republic of China and require nuclear weapons to end it.

The film was also produced at least one year before the first "hydrogen bomb" or megaton range explosive device was demonstrated to be possible, with test shot Ivy Mike in 1952. At least four years before the first true "hydrogen bomb" of the Soviet Union(USSR), the RDS-37, on November 22, 1955, which yielded 1.6 megaton. About 6 years before the first successful test launch of the world's first Intercontinental ballistic missile(ICBM), the Soviet Union's R-7 in 1957, up until this time strategic bombers were not but one of the three strategic nuclear triad delivery systems of the modern era but the sole intercontinental nuclear weapon delivery system capable of reaching the USA from the Soviet Union and vice versa.

Before advances in precision guided munitions and physics package miniaturization, "city busting"[11] or countervalue targeting, was the most likely nuclear war scenario, as effective counterforce warfare with nuclear weapons had yet to become all that conceivable, with the potential exceptions of being used in roles where a low degree of accuracy would not result in a waste of a bomb, such as destroying large military bases, aircraft carriers, Communist human wave attacks, massive mine fields in trench warfare(which was what the Korean War escalated into), radiological area denial and used at night prior to a frontal assault in the hopes of inducing widespread enemy troop flash blindness. Practically all other uses were likely to be ineffective against an enemy's military due to weather factors, poor munition accuracy and the unwieldy physics packages of the weapons of the era, making them generally unfit for the mobile battlefield.

It is now estimated that it was not until after 1957 that the USSR attained more than about 50 nuclear munitions in its nuclear weapons arsenal.[12][13] Almost all of this stockpile was likely designed to be air dropped by the Myasishchev M-4[14] and Tupolev Tu-4 bombers as strategic nuclear weapons on US and West European NATO cities respectively. As large numbers of small true tactical nuclear weapons only began to seriously populate the USSR's stockpile in the 1960s and beyond.

Assessment[edit]

1954 test shot Nectar of Operation Castle produced a yield of 1.69 megatons and was conducted off the coast of Teiter island. Note the distinctive near instantaneous double flash, with the second appearing brighter than the sun, and the blast wave slowly, by comparison, spreading out turning the calm ocean water a frothy white when it arrives. The maximum average nuclear fireball radius is approximately 1.4 to 1.6 km (0.87 to 0.99 mi).[15][16] The outdoor blast and flash burn LD50s would be around 8 and 12 km respectively.[15][17] Assuming personnel did not take any prompt countermeasures, instead standing still absorbing the entire light energy emitted over the ~20 seconds of emission.

Within a considerable radius from the surface of the nuclear fireball, 0–3 kilometerslargely depending on the explosion's height, yield and position of personnel—ducking and covering would offer negligible protection against the intense heat, blast and prompt ionizing radiation following a nuclear explosion. Beyond that range however many lives would be saved by following the simple advice, especially since at that range the main hazard is not from ionizing radiation but from blast injuries and sustaining thermal flash burns to unprotected skin.[17][18][19][20] Furthermore, following the bright flash of light of the nuclear fireball, the explosion's blast wave would take from first light, 7 to 10 seconds to reach a person standing 3 km from the surface of the nuclear fireball, with the exact time of arrival being dependent on the speed of sound in air in their area.[21] The time delay between the moment of an explosions flash and the arrival of the slower moving blast wave is analogous to the commonly experienced time delay between the observation of a flash of lightning and the arrival of thunder during a lightning storm, thus at the distances that the advice would be most effective, there would be more than ample amounts of time to take the prompt countermeasure of 'duck and cover' against the blast's direct effects and flying debris.[22] For very large explosions it can take 30 seconds or more, after the silent moment of flash, for a potentially dangerous blast wave over-pressure to arrive at, or hit, your position.[23]

It must be kept in mind that the graphs of lethal ranges as a function of yield, that are commonly encountered,[17][18] are the unobstructed "open air", or "free air" ranges that assume amongst other things, a perfectly level target area, no passive shielding such as attenuating effects from urban terrain masking, e.g. skyscraper shadowing, and so on. Therefore they are thus considered to present an overestimate of the lethal ranges that would be encountered in an urban setting in the real world.[24]

The Rest House of Hiroshima Peace Park, the basement of which Eizo Nomura was in on August 6, 1945 when Little Boy exploded overhead. The building was built as a Kimono Shop in 1929. It, like most reinforced concrete buildings in Hiroshima, remained standing, without windows, following the blast and firestorm.

To highlight the effect that being indoors, and especially below ground can make, despite the lethal open air radiation, blast and thermal zone extending well past her position at Hiroshima,[18] Akiko Takakura survived the effects of the 16 kt atomic bomb at a distance of 300 meters from ground zero, sustaining only minor injuries, due in greatest part to her position in the lobby of the Bank of Japan, a reinforced concrete building, at the time of the nuclear explosion,[25][26] and to highlight the protection conferred to an individual who is below ground during a nuclear air burst, Eizo Nomura survived the same blast at Hiroshima at a distance of 170 meters from ground zero.[27] Nomura, who was in the basement of what is now known as the rest house, also a reinforced concrete building,[27] lived into his early 80s.[28][29][30]

In contrast to these cases of survival, the unknown person sitting outside on the steps of the Sumitomo Bank next door to the Bank of Japan on the morning of the bombing—and therefore fully exposed—suffered what would have eventually been lethal third degree burns from the near instant nuclear weapon flash if they hadn't then promptly been killed by the slower moving blast wave, when it reached them, ~2 seconds later.[31]

To elucidate the effects on laying flat on the ground in attenuating a weapons blast, Miyoko Matsubara, one of the Hiroshima maidens, when recounting the bombing in an interview in 1999, said that she was outdoors and less than 1 mile from the hypocenter of the Little Boy bomb. Upon observing the nuclear weapons silent flash she quickly lay flat on the ground, while those who were standing directly next to her, and her other fellow students, had simply disappeared from her sight when the blast wave arrived and blew them away.[32][33]

Position of the body can have a considerable influence in protection from blast effects. Lying prone on the ground will often materially lessen direct blast effects because of the protective defilade effects of irregularities in the ground surface. Ground also tends to deflect some of the blast forces upward. Standing close to a wall, even on the side from which the blast is coming, also lessens some of the effect. Orientation of the body also affects severity of the effect of blast. Anterior exposure of the body may result in lung injury, lateral position may result in more damage to one ear than the other, while minimal effects are to be anticipated with the posterior surface of the body (feet) toward the source of the blast.[34]

The human body is more resistant to sheer overpressure than most buildings, however, the powerful winds produced by this overpressure, as in a hurricane, are capable of throwing human bodies into objects or throwing objects at high velocity, both with lethal results, rendering casualties highly dependent on surroundings.[35] For example, Sumiteru Taniguchi recounts that, while clinging to the tremoring road surface after the fat man detonation, he witnessing another child being blown away, the destruction of buildings around him and stones flying through the air.[36] Similarly, Akihiro Takahashi and his classmates were blown by the blast of little boy by a distance of about 10 meters, having survived due to not colliding with any walls etc. during his flight through the air,[37] Likewise, Katsuichi Hosoya has a near identical testimony.[38]

During the 2013 Chelyabinsk meteor explosion, a fourth-grade teacher in Chelyabinsk, Yulia Karbysheva, saved 44 children from potentially life threatening ballistic window glass cuts by ordering them to hide under their desks when she saw the flash. Despite not knowing the origin of the intense flash of light, she ordered her students to execute a duck and cover drill. Ms. Karbysheva, who herself did not duck and cover but remained standing was seriously lacerated when the explosion's blast wave arrived and window glass blew in severing a tendon in one of her arms, however not one of her students, who she ordered to hide under their desks, suffered a cut.[39]

Anything that can cast a shadow will protect that which is shadowed from being burnt. In this case a valve protecting a portion of the wall of a gas holder from having a line-of-sight with the nuclear fireball whereas all un-shadowed painted surfaces were lightened.[40] A large number of these permanent markings were used, by extrapolating backwards, to determine the exact point of detonation in the sky.[41][42][43]
A man who was present at Nagasaki on August 9, 1945 during the dropping of the 20 kiloton Fat Man bomb, this photo displays that 1st and 2nd degree burn injuries he experienced on his unclothed skin, the shoulder and arm, while the thin vest garment of clothing that he was wearing at the time of the explosion completely protected his stomach and lower chest from experiencing similar burns. A clearer color restored version of this picture is also available.[44]

The advice to cover one's exposed skin with anything that can cast a shadow, like the picnic blanket and newspaper used by the family in the film, may seem absurd at first when one considers the capabilities of a nuclear weapon, but even the thinnest of barriers such as cloth or plant leaves would reduce the severity of burns on the skin from the thermal radiation—which is primarily light rays in the ultraviolet, visible, and infrared range, and it is this combination of light rays that delivers the burning energy to exposed skin areas.[45] This burning thermal radiation would be experienced by people within range for several seconds after the explosion.[46] A photograph taken about 1.3 km from the hypocenter of the Hiroshima bomb explosion showed that the shadowing effect of leaves from a nearby shrub protected a wooden utilities pole from charring discoloration due to thermal radiation, while the rest of the telephone pole, not under the protection of the leaves, was charred almost completely black.[47]

The advice to "duck and cover" holds well in many situations where structural destabilization or debris may be expected, such as during an earthquake or tornado. At a sufficient distance from a nuclear explosion, the blast wave would produce similar results and ducking and covering would perhaps prove adequate. It would also offer some protection from flying glass and other small, but dangerous, debris. Ducking and covering would also reduce exposure to the gamma rays.[48] Since they are mostly emitted in a straight line, people on the ground will have more chance to have obstacles serving as radiation protection such as building foundations, cars, etc. between them and the source of radiation and scattered skyshine. The technique offers a small protection against fallout, however it must be said that the technique assumes that after the initial blast, a person who ducks and covers will realize when it is wise to cease ducking and covering (as the blast danger has passed) and to then seek out a more sheltered area, like an established or improvized fallout shelter to protect themselves from the ensuing local fallout danger. If such a shelter is unavailable, at this point the person is advised to follow the Shelter in Place protocol, or if given, evacuation advice. Evacuation orders would entail exiting the area completely by following a path perpendicular to the wind direction, and therefore perpendicular to the path of the fallout plume.[49] While "sheltering in place" is staying indoors, in a preferably sealed tight basement, or internal room, for a number of hours, with the oxygen supply available in such a scenario being more than sufficient for 3+ hours in even the smallest average room, under the assumption that the improvised seal is perfect, until carbon dioxide levels begin to reach unsafe values and necessitate room unsealing for a number of minutes to create a room air change.[50][51]

After all, "Duck and Cover" is a first response countermeasure only, in much the same way that "Drop, Cover and Hold On" is during an earthquake, with the advice having served its purpose once the earthquake has passed, and possibly other dangers—like a tsunami—may be looming.

In the era the advice was originally given, the most common nuclear weapons were weapons comparable to the US Fat Man and Soviet Joe-1 in yield. The most far reaching dangers that initially come from the nuclear explosion of this, and higher, yield weapons as airbursts, are the initial flash/heat and blast effects and not from fallout. This is due to the fact that when nuclear weapons are detonated to maximize the range of building destruction, that is, maximize the range of surface blast damage, an airburst is the preferred nuclear fuzing height, as it exploits the mach stem phenomenon. This phenomenon of a blast wave occurs when the blast reaches the ground and is reflected. Below a certain reflection angle the reflected wave and the incident wave merge and form a reinforced horizontal wave, this is known as the 'Mach stem' (named after Ernst Mach) and is a form of constructive interference and consequently extends the range of high pressure.[52][53][54][55] Airburst fuzing, as one would expect, increases the range that peoples skin will have a line-of-sight with the nuclear fireball. However as a result of the high altitude of the explosion, most of the radioactive bomb debris is dispersed into the stratosphere, with a great column of air therefore placed between the vast majority of the bomb debris/fission reaction products and people on the ground for a number of crucial days before it falls out of the atmosphere in a comparatively dilute fashion, this "delayed fallout" is henceforth not an immediate concern to those near the blast. On the other hand, the only time that fallout is rapidly concentrated in a potentially lethal fashion in the local/regional area around the explosion is when the nuclear fireball makes contact with the ground surface, with an explosion that does so, being aptly termed a surface burst.[56] For example in the Operation Crossroads tests of 1946 on Bikini Atoll, Test Able (an air burst) had little local fallout, but the infamous Test Baker (a shallow underwater burst) left the test targets badly contaminated with radioactive fallout.

Widespread radioactive fallout itself was not recognized as a threat amongst the public at large before 1954, until the widely publicized story of the 15 megaton surface burst of the experimental test shot Castle Bravo on the Marshall Islands. The explosive yield of the Castle Bravo device the Shrimp was unexpectedly high, and therefore correspondingly higher amounts of local fallout were produced. This resulted, months after the explosion, in the accidental death of 1 of the crew members of a Japanese fishing boat known as the Lucky Dragon, which was fishing outside the planned fallout zone.

It is however unlikely that a well funded belligerent with nuclear weapons would waste their weapons with fuzing to explode below or on the surface, as both test shot Baker, and Castle Bravo were respectively. Instead, to maximize the range of city blast destruction and immediate death, an air burst is preferred,[56] as the ~ 500 meter explosion heights of the only nuclear weapons used on cities, Little Boy and Fat Man also attests to.[57] Moreover, with air bursts the total amount of radiation contained in the fallout, in units of Becquerel, is somewhat less than the total that would be released from a surface or subsurface burst, as in comparison, depending on the height of burst, little to no neutron activation or neutron induced gamma activity of soil occurs from air bursts.[58] Therefore the initial danger from concentrated local/'early' fallout (which takes on the color of the soil around the fireball, commonly with a dusty pumice or ash-like appearance, as experienced by the crew of the Lucky Dragon) remains low in a global nuclear war scenario. Instead the fallout most likely to be encountered by most survivors in this scenario is expected to be the less dangerous global/'late' fallout.[59] As an air burst at optimum height will produce a negligible amount of early fallout.[60]

A notable comparison to underline this is found when one compares the 50 megaton airburst Tsar Bomba, which produced no concentrated local/early fallout, and thus no known deaths from radiation, with the surface burst of the 15 megaton Castle Bravo, which in comparison, due to the local fallout produced, killed one of the 23 strong crew of the Lucky Dragon and made the entire Bikini Atoll unfit for further nuclear testing until enough time elapsed and the intensity of the radiation field had decayed to acceptable levels. By 1958, a total of 23 nuclear devices were exploded on or near the atoll,[61] with the majority occurring after the 1954 Operation Castle series, resulting in a total of about 42 megatons of pure fission product fallout being generated around the atoll, this made permanent aboveground habitation without remediation unwise for a decade or so, it was thus resettled in 1968. The inhabitants lived there again from 1968-1978, abandoning the atoll in 1978. As of 2014, the Atoll has had infrequent inhabitants since the 1990s, mainly for tours, a return to permanent safe habitation would require locally produced and consumed plant food to be grown with fertilizer, or alternatively, only imported plant food to be eaten.[62][63]

An operation Doorstep mother and daughter Mannequin in an improvised basement lean-to shelter prior to testing in test shot Upshot-Knothole Annie. To Shelter-in-place in such an area would offer an adequate fallout radiation protection factor or "dose reduction factor" of 20 or more.[64]

Furthermore, regardless of if a nuclear attack on a city is of the surface or air-burst variety or a mixture of both, the advice to shelter in place, in the interior of well built homes, or if available, fallout shelters, as suggested in the film Duck and Cover, will drastically reduce ones chance of absorbing a hazardous dose of radiation.[65] A real world example of this occurred after the Castle Bravo test where, in contrast to the crew of the Lucky Dragon, the firing crew that triggered the explosion safely sheltered in their firing station until after a number of hours had passed and the radiation levels outside fell to dose rate levels safe enough for an evacuation to be considered.[66][67] The comparative safety experienced by the Castle Bravo firing crew served as a proof of concept to civil defense personnel that Shelter in place(or "buttoning up" as it was known then) is an effective strategy in mitigating the potentially serious health effects of local fallout.[66]

The minimum typical protection factor of the fallout shelters in US cities is 40 or more, in many cases these shelters are nothing more than the interior of pre-existing well built buildings that have been inspected, and following their protection factors being calculated, re-purposed as fallout shelters.[68] A protection factor of 40 or more means that the Radiation shielding provided by the shelter reduces the radiation dose experienced by at least 40 times that which would be experienced outside the shelter with no shielding. "Protection factor" is equivalent to the modern term "dose reduction factor".[69]

In units of Rads, a comparison of the free air/unshielded fallout gamma dose and dose rate contours for a 2 megaton land surface burst detonation, with 1 megaton of the yield coming from fission reactions. Because of radioactive decay, the dose rate contours (on the right) contract after fallout has arrived, but the total absorbed dose contours (left) continue to grow. A similar color version of the right panel, dose rate contour plot, for a 0.1 to 10 kiloton surface burst is also available.[70]

During the first hour after a nuclear explosion, radioactivity levels drop precipitously. Radioactivity levels are further reduced by about 90% after another 7 hours and by about 99% after 2 days.[49] An accurate rule of thumb for approximating the radioactive dose rate produced by the decay of the myriad of isotopes present in nuclear fallout is the "7/10 rule". The rule states that for each 7 fold increase in time the dose rate drops by a factor of 10.[71] For example, assuming the fallout process has ended and the dose rate is a lethal in one hour exposure, 500 roentgens per hour, at one hour after detonation, then 7 hours after detonation the rate will be 50 R/hr, 49 hours after detonation (7×7 hours) the dose rate will be 5 R/hr, 343 hours after detonation (49×7—or about 2 weeks) the dose rate will be about 0.5 R/hr, at which point no special precautions would need to be taken and venturing outside into that dose rate for an hour or two would pose a close to negligible health hazard,[72] thus permitting an evacuation to be done with acceptable safety to a known contamination free zone. Following a nuclear detonation approximately 80 percent of the fallout would be deposited on the ground during the first 24 hours.[23]

Expert advice published in the 2010 document Planning Guidance for Response to a Nuclear Detonation is to shelter in place, in an area away from building fires, for at least 1 to 2 hours following a nuclear detonation and fallout arriving,[73] with the greatest benefit, assuming personnel are in a building with a high protection factor, is sheltering for no less than 12 to 24 hours before evacuation.[22] Therefore, sheltering for the first few hours can save lives.[74] Indeed fallout deaths are regarded by experts as the most preventable of all the effects of a nuclear detonation, being simply dependant on if personnel know how to identify an adequate shelter when they see one and enter one quickly, with the number of potential people saved being cited as in the hundreds of thousands.[75][76]

Some historians have thus far sought to dismiss civil defense advice as mere propaganda, despite detailed scientific research programs laying behind the much-mocked UK government civil defense pamphlets of the 1950s and 1960s, including the prompt advice of ducking and covering.[77]

The exercises of civil defense are seen by Historian Guy Oakes as having less practical use than psychological use: to keep the danger of nuclear war high on the public mind, while also attempting to assure the American people that something could be done to defend against nuclear attack.[78]

In U.S. Army training, soldiers are taught to fall immediately down, covering face and hands in much the same way as is described by the advice to duck and cover.[79]

Tornadoes[edit]

Students participate in a tornado drill, lining up along an interior wall and covering their heads. Tornado drills are an important element in tornado preparedness. Similar to other evidence based safety drills, they teach effective countermeasures and therefore increase survival rates if/when a tornado hits.

Ducking and covering does have certain applications in other, more natural disasters. In states prone to tornadoes, school children are urged to "duck and cover" against a solid inner wall of a school, if time does not permit seeking better shelter—such as a storm cellar—during a tornado warning. The tactic is also widely practiced in schools in states along the West Coast of the United States, where earthquakes are commonplace. Ducking and covering in either scenario would theoretically afford significant protection from falling or flying debris.

Earthquakes[edit]

In an earthquake, people are encouraged to "drop, cover, and hold on": to get underneath a piece of furniture, cover their heads and hold on to the furniture. This advice also encourages people not to run out of a shaking building, because a large majority of earthquake injuries are due to broken bones from people falling and tripping during shaking. While it is unlikely that "drop, cover and hold on" will protect against a building collapse, buildings built in earthquake-prone areas in the United States are usually built to earthquake "Life Safety" codes,[citation needed] and a building collapse (even during an earthquake) is rare. "Drop, cover and hold on" may not be appropriate for all locations or building types, but the red cross advises,[80] it is the appropriate emergency response to an earthquake in the United States.

See also[edit]

References[edit]

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  40. ^ Original caption: "Shadow" of band valve wheel on paint of a gas holder at Hiroshima. Radiant heat instantly burned paint where the heat rays were not obstructed. 6,300 feet from ground zero (Japanese photo). United States Strategic Bombing Survey, The Effects of Atomic Bombs on Hiroshima and Nagasaki (United States Government Printing Office: Washington, 1946) Chapter 3.
  41. ^ Kerr, George D.; Young, Robert W.; Cullings, Harry M.; Christy, Robert F. (2005). "Bomb Parameters". In Robert W. Young, George D. Kerr. Reassessment of the Atomic Bomb Radiation Dosimetry for Hiroshima and Nagasaki – Dosimetry System 2002. The Radiation Effects Research Foundation. pp. 42–43. 
  42. ^ Malik, John (September 1985). "The Yields of the Hiroshima and Nagasaki Explosions". Los Alamos National Laboratory. Retrieved March 9, 2014. 
  43. ^ Malik (1985) describes how various values were recorded for the B-29's altitude at the moment of bomb release over Hiroshima. The strike report said 30,200 ft, the official history said 31,600 ft, Commander Parson's log entry was 32,700 ft, and the navigator's log was 31,060 ft—the latter possibly an error transposing two digits. A later calculation using the indicated atmospheric pressure arrived at the figure of 32,200 ft.
    Similarly, several values have been reported as the altitude of the Little Boy bomb at the moment of detonation. Published sources vary in the range of 1,800 to 2,000 ft (550 to 610 m) above the city. The device was set to explode at 1,885 ft (575 m), but this was approximate. Malik (1985) uses the figure of 1,903 ft (580 m) plus or minus 50 ft (15 m), determined after data review by Hubbell et al (1969). Radar returns from the tops of multistory buildings near the hypocenter may have triggered the detonation at a somewhat higher altitude than planned. Kerr et al (2005) found that a detonation altitude of 600 m (1,968.5 ft), plus or minus 20 m (65.6 ft), gave the best fit for all the measurement discrepancies.
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  79. ^ "field manual FM 3-4 Chapter 4. "Dropping immediately and covering exposed skin provide[s] protection against blast and thermal effects."..."Immediately drop facedown. A log, a large rock, or any depression in the earth's surface provides some protection. Close eyes. Protect exposed skin from heat by putting hands and arms under or near the body and keeping the helmet on. Remain facedown until the blast wave passes and debris stops falling. Stay calm, check for injury, check weapons and equipment damage, and prepare to continue the mission."". 
  80. ^ "Drop, Cover, and Hold On – American Red Cross drill (pdf file)". 

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