Therefore, the Schwarzschild criterion dictates whether an element of a star will rise or sink if displaced by random fluctuations within the star or if the forces the element experiences will return it to its original position.
For the Schwarzschild criterion to hold the displaced element must have a bulk velocity which is highly subsonic.
In order to keep rising or sinking in the star the displaced element must not be able to become the same density as the gas surrounding it.
In order for this to be true it must move fast enough for there to be insufficient time for the element to exchange heat with its surroundings.
Stellar-structure models indicate that the two gradients are seldom of the same order of magnitude, so that the smaller can usually be neglected, even if both are always present.