Van Wagner's crown-fire initiation criterion asks one question of a surface fire: is it throwing enough heat upward to ignite the lowest live foliage? It compares the surface fire's Byram fireline intensity against a critical intensity that grows with the canopy base height — the distance from the ground to the bottom of the live crown — and with the foliar moisture content. Written compactly, the critical surface intensity is \( I_0 = (C \, h \, M)^{3/2} \), where \( h \) is canopy base height, \( M \) is a foliar moisture term and \( C \) is an empirical constant. When the surface fire's intensity clears \( I_0 \), the fire transitions into the crowns and its rate of spread changes discontinuously. This is why pruning lower limbs and reducing surface fuel are the same intervention seen from two sides: one raises \( h \), the other lowers the intensity that has to be beaten. The confusion it resolves is that crown fire is not a hotter version of surface fire; it is a threshold crossing, so small changes in the stand can flip the outcome. After this Concept you can decide whether a modelled surface fire is about to become a crown fire.
Video tutorial: Van Wagner Model for Crown Fire Initiation