The dependence of time to ignitionti, on heightxof a specimen is examined for radiant ignition of vertical cellulosic solids. There is a transitional irradiance value of about 4 cal/cm2sec, below whichti, decreases asXincreases and above whichti, is essentially independent ofX. In the present paper this phenomenon is explained on the basis of the time ts, required for establishment of a steady-state free-convection boundary layer. At low irradiance,ti, ≥tss, andx-dependent heat losses through the boundary layer cause the top of the panel to be hotter than the bottom and cause the pyrolysis rate and ignition time to depend onX. At high irradiance,ti, ≤tss, and ignition occurs before boundary-layer development, thereby causingti, to be independent ofX.