stack's description is fairly accurate - UA acts like a current, because it pretty much is one, usually of aether.

Electrical current is defined as dQ/dt. So if we presume an aether current, then we devise: dA/dt, where A=the aether distribution.

The question then becomes what is the exact distribution of the aether? In order to align with the Michelson-Morely experiments, such observations denote a constant aether wind. In this case, in alignment with Zetetic principles, the rate of change of the aether wind would vanish: dA/dt=0, precisely because the aether wind is constant. Hence, dA/dt=0, rather than a=g, as we would wish to show for a constant UA.

The upshot is another method may be needed to demonstrate a constant UA given observations of an aether wind..