Effects of Thermal Radiation
(1) Human damage

The fireball that developed in the air simultaneously with the detonation attained a maximum temperature of 300,000 degrees Centigrade at the instant of the explosion, a diameter of about 180 meters and a surface temperature of about 1,700 degrees; 1/100 of a second later, a surface temperature of about 7,000 degrees - ... - rising again; 0.3 second later, a maximum diameter of about
280 meters and surface temperature of about 5,000 degrces; 1 second later, a surface temperature of 1,700 degrees again; 3 seconds later, and thereafter, it gradually dropped. Ninety-nine percent of the thermal radiation from the fireball had strong effect on the ground only during the time from 1/100 to about 3 seconds after the detonation. The thermal radiation of ultraviolet rays caused fire, and loss of eyesight and heat burns on the human body. Thermal radiation heat burns of the exposed human skin were observed to about 3.5 kilometers from the hypocenter.

a.Within one-kilometer radius from the hypocenter, most of those v~ho sustained fatal heat burns died on the day of exposure or in a few days not only with their skin layers totally burnt out but also their intestines and other internal organs seriously ruptured. b.Within two-kilometer radius from the hypocenter, agreat number of people suffered serious heat burns c. Heat burns of those outside the radius of 3 kilometers of the hypocenter were comparatively light and limited to the surface of the skin.

The temperature of the heat rays in the hypocenter is estimated to have been from 3,000 degrees Centigrade to 4,000 degrees Centigrade, which may be compared with the melting point of an iron bar, 1,5360 C. Anythi~g near the hypocenter, both human and other materials was supposedly tumed to ashes. Persons located without shielding within about 1.2 kilometers from the hypocenter sustained fatal heat burns and 20% to 30% deaths were due to this injury.

(2) Damage done to materials

The uniqueness of thermal radiation effects on materials was vividly shown in such phenomena as spontaneous combustion, charring, and the imprinting of shadows of things.
Since there was spontaneous combustion on clothes of persons outdoors or laundry drying in the sun about 1.8-2 kilometeis from the hypocenter, the vast range of spontaneous combustion due to the direct thermal radiation may be easily conceived.
Railroad ties and wooden fences along railroads even 2.1 kilometers from
the hypocenter spontaneously ignited and burnt. The surface of granite stones within one kilometer of the hypocenter melted in the heat. Glass-like bubbles formed on the surface of roof-tiles within 600 meters of the hypocenter.
There was also spontaneous combustion on plants and trees. Many old and huge trees were left with their insides burnt.

Shadow of valve imprinted on a gas tank: Coal tar on the surface of the tank absorbed enough heat to make a clear shadow on the tank even though there was some distance between the valve and the side of the tank. The tar completely melted away. (2,000 meters from the hypocenter. Photographed by Shunkichi Kikuchi.)