Determination of the thickness δ of the stagnant zone, (example for Ra _∗  = 10⁷, Le = 0.01, Pr = 0.1). The steep curve shows the heat flux in the stagnant zone as a function of the assumed value of δ, the shallow curves the corresponding flux in the overturning zone. Intersection points are possible values of δ for stationary heat flow. Top: density ratio R_ρ = 4. For the higher density ratio of 5.5 (bottom), there is no value of the thickness for which the two match (see text).