9. About 35 per cent of the total energy released in an air burst and. about25 per cent of that in a surface burst wilf be enitted. as themaf radiation (l_ight aud heat) vithin the first minute folloving the explosion. r{ost of this energy wir.lbe released in the first few seconds. 10. The composition of this thernal radiation is quite similar to that of the sunwith some ultraviolet but mostly visible and infra-red 1ig.ht. I{ence the firstnotlceable sign that a nuclear air burst has taken place vi11 to an observer at somedistance be a very bright flash or "lightning" and - depending on the distance - aInore or less intense wave of heat, For a surface burst" the themal radiation thatreaches an object at some distance from ground zero will be less than for a l-ow airburst since part of the thernxal energy of a surface burst is absorbed by the earth,roch or wat er at ground zero, and part of it is shielded by surrounding terrainirregularities. For a high aLtitude buxst, j.e. above 30 km (]OO"0O0 ft.), thernxaleffects on the ground can be ignoredl except fo" very high yield weapons of the o"derof 10 Mt. or more. 11. I,leather and terrain conditions rail1 affect the amount of therma.l energyreflected or absorbed by the atr0osphere consid.erably. Fog, smoke oI hearxf cl-oualswilf substantially reduce the amount of thermal radiation that ,nder clearatmospheric conditions woul-d reach an object. On the other hand reflecting surfaces " such as snow of ice, will enhance thermal "adiation. ltre finally note here that forunderground or underr^ra,ter bursts therx0al energy virl on-Ly be effective very c.lose tothe firebau, since if there is no surface breakthrough the heat will rapidly be absorbed by the surrounding earlh or water.^/ 351392 E3glish Anne:t Page I52 Di stance kmuDDer curves: Ito Uirl imi tea visibilitYf I I
**Analysis**
This text appears to be discussing the effects of nuclear explosions on the environment and human perception, specifically the thermal radiation emitted during the initial minutes following the explosion. The text also touches on the factors that affect the amount of thermal energy that reaches the ground, such as atmospheric conditions, terrain, and the altitude of the burst. Given the technical and scientific nature of the content, it seems likely that the text is related to a field of study that is not directly related to human activities or economic development.

**Category**
Science and Technology