I had used the bomb icon as a joke in [this][1] computer/IT question. When I got a ping back this morning, it just occurred to me that almost all images I've seen of the (in)famous mushroom cloud show a vertical column rising perpendicular to the ground and a horizontal planar ring parallel to the ground. 

![enter image description here][2]

Not that I'm an expert (that's why this question) but I have rarely seen anything go in the 45 degree angle. Or for that matter anything other than the 'special' 0 degree horizontal plane and 90 degree vertical column. Shouldn't there be radial vectors at all angles between 0 and 90 degrees giving rise to a hemispherical explosion envelope? Why is it a vertical cylinder? 

PS: I understand the top expands eventually on cooling and lowered air pressure giving the mushroom look but my questions is for the previous stage - the vertical column.


  [1]: http://superuser.com/questions/488322/permanently-remove-disable-touch-keyboard-in-windows-8-taskbar
  [2]: https://i.sstatic.net/yqTdB.jpg