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32 DESCRIPTIONS OF NUCLEAR EXPLOSIONS8014;:: 70ww -l'- 12 ~:t> 60 ~~00 0 wzct 5010 I-::>00 I-::> ~0:I: I- 800~~ 40 00 ::>::>91 30916 uU:t> l-I- 20 4 ~:I: -(!) w-:I:W:I: 10 2l'-00 00 I 2 3 4 5 6TIME AFTER EXPLOSION (MINUTES)Figure 2.12.Height of cloud top above burst height at various times after a I-megatonexplosion for a moderately low air burst.2.15 The cloud attains its maximum the height curve in the range of aboutheight after about 10 minutes and is then 20- to lOO-kilotons TNT equivalent issaid to be "stabilized." It continues to due to the effect of the tropopause ingrow laterally, however, to produce the slowing down the cloud rise. For yieldscharacteristic mushroom shape (Fig. below about 15 kilotons the heights in-2.15). The cloud may continue to be dicated are distances above the burstvisible for about an hour or more before point but for higher yields the values arebeing dispersed by the winds into the above sea level. For land surface bursts,surrounding atmosphere where it the maximum cloud height is somewhatmerges with natural clouds in the sky. less than given by Fig. 2.16 because of2.16 The dimensions of the stabi- the mass of dirt and debris carried aloftlized cloud formed in a nuclear explo- by the explosion.sion depend on the meteorological con- 2.17 For yields below about 20 kiditions,which vary with time and place. lotons, the radius of the stem of theApproximate average values of cloud mushroom cloud is about half the cloudheight and radius (at about ]0 minutes radius. With increasing yield, however,after the explosion), attained in land the ratio of these dimensions decreases,surface or low air bursts, for conditions and for yields in the megaton range themost likely to be encountered in the stem may be only one-fifth to one-tenthcontinental United States, are given in as wide as the cloud. For clouds whichFig. 2.] 6 as a function of the energy do not penetrate the tropopause the baseyield of the explosion. The flattening of of the mushroom head is, very roughly,