  The rotation of granules shows thermal properties similar to the thermal
motion of molecules, which effects convection criterion and solar granule
size.The granular rotation generates the rotational additional pressure and
corresponding rotational equivalent temperature in the spherically symmetric
expansion of the granule.The rotation equivalent temperature represented by the
rotational speed and the granular size is combined with the Schwarzschild
criterion to obtain a new convection criterion determined jointly by the
granular size, temperature gradient, and rotational speed gradient.In most of
the solar convection zone, the convection is driven by the temperature gradient
and suppressed by the rotational speed gradient.In the solar quiet region, the
larger the granular size, the weaker the convection.The small granules with the
sizes smaller than the critical size are in natural convection, while the large
granules with the sizes larger than the critical size are in forced
convection.In the solar active region, the larger the rotational speed, the
smaller the critical size and the temperature.So sunspots have smaller granules
and lower temperatures.

Solar and Stellar Astrophysics;Fluid Dynamics