The effective inflow layer is the depth of air in a sounding that has enough buoyancy and little enough inhibition to actually feed a thunderstorm. It replaces the assumption that a storm draws only from the ground or from a single fixed pressure level.
How it is defined
Thompson, Edwards, and Hart defined the layer using two thresholds applied to lifted parcels through the sounding. A parcel qualifies if it has at least 100 J per kg of CAPE and no more than 250 J per kg of convective inhibition. The effective inflow layer is the first contiguous band of parcels that satisfy both criteria.
The base of the layer is the lowest qualifying parcel. The top is the highest qualifying parcel above that base before the criteria fail. In a surface-based warm sector, the base is often at or very near the ground. Above a cool boundary layer, or beneath a warm front, the base may sit hundreds of meters above the surface, and the layer represents the elevated air an above-boundary storm would actually ingest.
How forecasters use it
The effective inflow layer's depth is a first-look diagnostic on its own. A deep layer, several kilometers thick, indicates a broadly favorable column for sustained deep convection. A thin or missing layer, especially with a base well above the surface, indicates that inflow is limited to a narrow band of air and that surface-based supercells are unlikely.
The layer is also the base for the effective bulk shear and the effective storm-relative helicity that dominate modern supercell checklists. On the Storm Prediction Center sounding display, forecasters read the base and top of the layer together with the shear and helicity calculated across it, all as part of one coherent picture of what the environment supports.
Important limits
The 100 J per kg CAPE and 250 J per kg CIN thresholds are calibrated guides drawn from a large observational database of supercells. They are not physical constants. In marginal environments, small changes in the base state can shift the diagnosed layer significantly, and the same sounding computed slightly differently can produce different effective-layer values.
The layer also assumes lifted parcels. Real inflow may include mixing with drier air, entrainment losses, and forced ascent that a pure parcel calculation does not capture. Two soundings with identical effective inflow layers can produce very different storms if one has strong entrainment or a shallow moist tongue that the calculation cannot see.
Why the layer replaced fixed assumptions
Older supercell diagnostics used fixed-depth parameters, most commonly the surface-to-6 km bulk shear and the surface-to-3 km storm-relative helicity. Those choices work well when the storm is surface-based and tall, but degrade when the storm is elevated or shallow. Elevated supercells over cool near-surface layers, and cool-season supercells with tops below 8 km, were poorly served by fixed layers.
By moving the base of the calculation up when the near-surface air cannot feed the storm, and by scaling the top of the calculation to the storm's actual depth, the effective inflow layer allows a single parameter set to describe surface-based, elevated, deep, and shallow supercells with comparable accuracy. That unification is why the modern SPC mesoanalysis and NSHARP tools default to the effective-layer versions.