ay tegion and is strocgly absorbed by the ambient air. The growth of the ses level fireball is priscipally by hydrodyaxaics which form wery cuicaly and carry sway most of the energy yield of the weapon. Very little enerzy es- capes the fireball before formation of the shock fronr which absorbs the vadiant energy of the inper fireball, thereby decreasing the effective brightness of the X-ray beated air. After breakaway, the expanding shock becomes transparent and a second aaximm in the thermal pulse occurs which contains essentially all the energy radiated from the burst. The fraction of the yield contained in the thermal pulse is about one-third of the total yield for a sea level explosion. + Because of lower air density, X-rays is deposited in a larger volume at higher altitude. The air tesperature of the fireball is then somewhat lower than at sea level, and more of the radiant energy is in the spectral region for which cold air is transparent. Bydrodynamic phenomena do not form as quickly as at sea level, so that more energy is radiated before the thermal minimm. This enhancement of the radiation constitutes a loss of energy which would otherwise be carried sway hydrodynasically. The result is that bydrodynamics become less important and thermal radiation is increased with altitude. Depending on yield, there is an altitude at which there is no second maxizam in the thermal pulse. ton weapons. This occurs below 200,000 feet for mega- At still greater altituces, the portion of the yield that is in thermal radiation begins to decrease again. This is caused by X-ray deposition which is so diluted spatially that the fireball] air is not very bot and cannot radiate effectively. 11.2.1 Crexge Shot Orang S$ not expected to be substantially different from sea level shots in sany respects. At this altitude, X-rays are still absorbed in a rather small region around the bomb. Hydrodynaazic phenrca- ena develop somewhat later than at sea level but still constitute the saost isportant mchanisa for transporting energy away from the explosion. As at sea level, a minim in the thermal puise occurs, although later than at sea level, and relatively more radiation escapes in the early stages of the explosica before the miniam. Only a slightly larger fraction of tke yield is radiated in comparison to that at sea level. 333