Change With Advected Air MET

Very broadly speaking, if temperature falls faster with height that becomes more unstable (less stable) and if temperature falls slower with height that becomes less unstable (and more stable.)

Now think of the advection of air. If cold air is advected aloft or warm air is advected below, that means a greater temperature change with height and therefore is is less stable. Conversely, If cold air is advected at the surface or warm air is advected aloft, that means a smaller temperature change with height and therefore is is more stable.