As described in the Fundamental Dynamics section, a pool of warm air aloft over a cold pool at the surface produces lower pressure at mid levels and higher pressure at the surface. The flow field responds by diverging at the surface and converging at mid levels. The flow that converges in from the rear of the system at mid levels is known as the rear-inflow jet (RIJ). As shown in the graphic to the right, the convergence from the front of the system tends to be blocked by the updraft, so most of the flow converges in from the rear of the system.