The purpose of this report is to define exterior kindling fuels and present their distribution in urban areas, to summarize conditions which determine their ignition by atomic weapons, and to consider these primary ignitions in the perspective of the mass-fire-problem. Solid wood surfaces and other common building materials are not ignited directly by atomic bomb thermal radiation beyond the range of severe blast damage. Only certain kinds of materials, called "kindling fuels," which are either less than 1/100-in. thick like newspapers or grass, or of low density such as rotted wood will be ignited. Not all kindling fuel cumulations are potential sources of primary fires. They must be exposed directly to the fireball and must be sufficiently large to burn long enough for fire to spread and ignite other combustibles. Kindling fuel cumulations which meet these two conditions are considered to be "primary ignition points." Primary ignition point distributions in transient exterior fuels were surveyed by urban land-use-classes: slum residential, poor residential, good residential, large manufacturing, small manufacturing, wholesale distribution, downtown retail distribution, neighborhood retail distribution, and water front. These surveys by use-classes show surprising similarity in ignition point distribution between large cities, and indicate that ignition point distribution is closely correlated with urban land use. Slum and wholesale use-classes contained 10 to 12 times the number of ignition points found in good residential and manufacturing areas. Newspapers constitute more than 60 per cent of the transient exterior kindling materials found in urban areas and will produce more than 10,000 primary ignitions per square mile in slum residential and wholesale use-class areas where total thermal energy from atomic weapons is 3 cal/sq cm or greater and relative humidity is 20 per cent or less. Most other exterior kindling materials will ignite when total thermal energy is less than 5 cal/sq cm. Ignition energies may be corrected for changes in fuel moisture by applying a correction factor correlated with relative humidity.

Misc. No.: AFSW-TR-413. Accession No.: 55612. Ntis: ad018183.