A photograph taken about 1.3 km from the hypocenter of the Hiroshima bomb explosion showed that the shadowing effect of leaves from a nearby shrub protected a wooden utilities pole from charring discoloration due to the burst of thermal radiation, however the rest of the telephone pole, not under the protection of the leaves, was charred almost completely black. The difference in required flash-energy necessary to produce essentially immediate, though transitory, non-propagating flaming, and that required to achieve a continued self-sustained propagating flaming are orders of magnitude in difference for most combustible materials. In the case of untreated timber it is largely dependent on the depth of char. While the propagating fires in both Japanese cities were almost exclusively ignited by the blast wave overturning charcoal cooking-braziers and similar secondary events, thermal flash-fires from untreated fabric and timber in the urban environment is considered potentially the widest destructive effect of the higher yield explosive devices.