From what I read, the bomb exploded 600 meters above ground. There was a massive spherical explosion. Tons of brown and black debris were created on the ground as a result. The heat of the explosion rose, the water around it condensed, it pulled cold air up from underneath it like a vacuum, along with a lot of the debris from the ground. The white column and the mushroom cloud are composed of condensed water, and the brown column is the debris of the city being pulled up from the rising heat of the explosion.