Examples of the dependence of T₂₁ on δ and V_bc (each in units of its cosmic standard deviation; see Section 2.1). The points show the outputs of the hydrodynamical simulations, and the curves show our interpolation. We show the case with clumping (solid curves) and without, i.e., uniform simulation boxes without small-scale fluctuations (dashed curves); the latter case is independent of V_bc and the values are shown as V_bc = 1.