Convective cells form as fluid is heated from below. As the
fluid near the bottom warms, its density decreases and buoyancy
causes it to rise while cooler fluid descends to replace it.
This fluid motion due to temperature gradients is called
Rayleigh-Benard convection and the cells in which the motion
occurs are called Benard cells. This particular type of
convection is essentially what happens when a pot is placed on a
hot stove, so the shapes are familiar. Similar shapes also form
on the sun’s photosphere, where they are called granules.