Its due to the [WP-Rayleigh–Taylor instability][1]: it is an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid.  
Hot hair raises and colder goes down.  
A Mushroom cloud formed by hot wet air :  
![enter image description here][2]  
google for images or videos of 'Rayleigh–Taylor'  


  [1]: http://en.wikipedia.org/wiki/Rayleigh%E2%80%93Taylor_instability
  [2]: https://i.sstatic.net/T5Ze5.jpg