The initial bubble is 0.5 degree Centigrate warmer than the environment. The size of the domain is 1000mts height by 1000 mts width. A grid size of dx=dz=5mts. and dt=0.01 sec and second order diffusion of 0.3 m*m/s. The change in color is because, I did not fixed the scale for all the frames in ferret.