Flame sag, a phenomenon observed as flame sinks downwards along the leeward sidewall, occurs for a pool fire having its rim above the ground in cross flows. Few data or investigations of this phenomenon have been reported in the literature. In the present study, the length of flame sag measured from the lowest point of sunk flame to the pool rim; as well as the critical speed of the cross flow under which the lowest point of sunk flame reaches the ground level (namely flame sag length equaling sidewall height), were quantified. Experiments were carried out by employing square quartz sand box pool fires with dimensions in the range of 10-20 cm for various cross flow air speeds, heat release rates and pool rim heights. Propane is used as the fuel. The results showed that the length of the flame sag increased with the increasing of the fire source heat release rate. And it also increased with the increasing of the air speed of the cross flow. However, the flame sag length decreased with increasing pool size. The critical air speed of the cross flow at which the flame sag reached the ground increased with increasing pool size and rim height, meanwhile decreased with increasing heat release rate. A scaling analysis for the interaction physics of the fuel flow near the base with the cross flow, and the total buoyant flow (heat release rate) of the fire with the cross flow, was provided to characterize the flame sag behavior. Two dimensionless quantities [(rho(alpha) - rho(f))gD/rho U-alpha(w)2], [((Q) over dot / D)g/rho(alpha)c(P)T(alpha)U(w)(3)]namely an inverse Froude number for the base and an inverse Froude number for the interaction of total buoyancy of the fire with cross flow, were derived from the analysis with a new length scale found as xi(L) = rho U-alpha(w)2/(rho(alpha) - rho(f))g. The data of flame sag length, as well as the critical value of the cross flow speed for flame sag reaching ground level, were shown to correlate well to the proposed dimensionless quantities.