Fig. 3 Dependence of the ratio t_mh/t_m on z₀/ for dual probes near a surface or front. In the heterogeneous system the sensor maximum temperature occurs at t = t_mh. For a homogeneous system it occurs at t = t_m. Here is the distance between probes, z₀ the distance between the surface or front and the probes (Configuration [a]) or the nearer probe (Configurations [b] and [c]). The near-surface curves apply also behind a wetting front