Current uncertainties regarding interactions of blast waves on fires preclude reliable estimates of the outcome of a nuclear attack on the United States. This study focuses on the threshold air blast condition for extinguishing fires initiated by thermal radiation. Flame blowout tests with simulated nuclear air-blast waves were conducted in the SRI-developed shocktube facility on class-B fires both in a flat plate geometry and stabilized behind barriers. The results support the concept of flame displacement as a mechanism of extinguishment. Data on class A fuels in practical configurations are needed next.