Step away, and the line looks sharp and crisp. The next time you have an opportunity to fly, observe the clouds as you pass through them. You will notice that the edges of a cloud are often diffuse. Just try to stay out of the cumulonimbus cloud, whose distinctive boundaries could give your airplane a sharp bounce. When a bubble gets large enough, the water's surface tension can no longer hold it, and so it rises.
Fluids having different densities behave quite independently.
The bubble stays a bubble all the way to the top where it breaks free as steam. So, too, with clouds. A spot on the earth's surface gets hotter than the surrounding area. An example would be the black, flat tarred roof of a large building or a vacant parking lot. The air above it heats up and forms a bubble of hot air, which is less dense than the surrounding air.