推荐回答（2个）
这个雾叫做“音爆云”。音爆云的确只在物体作跨音速运动的时候出现（也就是由亚音速进入超音速这个阶段）。但是，并不是所有的物体在任何情况下都可以出现音爆云。
在解释子弹没有音爆云之前，我先解释下音爆云的形成原理：当飞机接近音速时，将会逐渐追上自己发出的声波。此时，由于机身对空气的压缩无法迅速传播，将逐渐在飞机的迎风面及其附近区域积累，最终形成空气中压强、温度、速度、密度等物理性质的一个突变面——激波面（或称震波，Shock Wave）。这个激波面，是声学能量的高度集中面，所以又称音锥。
当飞机由亚音速进入到超音速时，激波面后方以气压骤降导致温度也极具降低，引起水气凝结导致出现了以飞机为中心轴、从机翼前段开始向四周均匀扩散的圆锥状云团,这就是普朗特-格劳厄脱凝结云效应（Prandtl-Glauert condensation clouds），也就是我们知道的“音爆云”。
当然，大部分音爆云形成在刚好加速穿过了音障速度临界的一霎那，不过在合适条件下，接近音速的飞机也可以产生音爆云。
所以，音爆云的形成需要
1：速度在跨音速阶段
2：空气密度大的环境，换句话说就是飞行高度越低越好。这有利于增大激波面在气压和温度上的强度反差。
3：潮湿空气更容易形成音爆云
这就解释了为什么音爆云多出现在海面作低空加速飞行的战斗机身上。
至于子弹，步枪子弹速度在离开枪口之前就已经达到或超过了音速，明显不具备形成音爆云环境的条件。
对于速度和音速差不多的手枪、气枪子弹，由于子弹的空气阻力比飞机小得多，更降低了出现音爆云的几率。再者，越过海面的飞机可是个庞然大物，如果出现音爆云很容易看到
；而子弹就这么小一点，就算满足条件出现了音爆云，那么快的速度下肉眼也很难分辨，需要借助摄影仪器慢速播放才能看见。
你看到的那张照片是在海面上飞行的飞机吧？
因为水汽的关系是你可以看到那样的雾，实际上这样的气流波动也是有的，只是如果不借助设备看不到罢了。
你去看看人家用高速摄影技术拍的子弹出膛，也会发现类似的气流的。
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