For an explosion in the atmosphere, the fireball quickly expands to maximum size and then begins to cool as it rises like a balloon through buoyancy in the surrounding air. As it does so, it takes on the flow pattern of a vortex ring with incandescent material in the vortex core as seen in certain photographs. This effect is known as a mushroom cloud. Sand will fuse into glass if it is close enough to the nuclear fireball to be drawn into it, and is thus heated to the necessary temperatures to do so; this is known as trinitite. At the explosion of nuclear bombs lightning discharges sometimes occur.