front move at lower velocities. This velocity, which is called the “particle velocity,” is associated with the “dynamic pressure,” or the pressure formed by the winds produced by the shock front. As the shock front expands into increasingly larger volumes of the medium, the incident pressure decreases and, generally, the duration of the pressure-pulse increase. If the shock wave strikes a rigid surface (e.g., a building) at an angle to the direction of the wave's propagation, a reflected pressure is instantly developed on the surface and this pressure rises to a value that exceeds the incident pressure. This reflected pressure is a function of the incident wave's pressure and the angle formed between the rigid surface and the plane of the shock front. Fragments An important consideration in the analysis of the hazards associated with an explosion is the effect of any fragments produced. Although fragmentation most commonly occurs in high explosives events, fragmentation may occur in any incident involving ammunition and explosives (A&E). Depending on their origin, fragments are referred to as “primary” or “secondary” fragments. Primary fragments result from the shattering of a container (e.g., shell casings, kettles, hoppers, and other containers used in the manufacture of explosives and rocket engine housings) in direct contact with the explosive. These fragments usually are small, initially travel at thousands of feet per second, and may be lethal at long distances from an explosion. Secondary fragments are debris from structures and other items in close proximity to the explosion. These fragments, which are somewhat larger in size than primary fragments and initially travel at hundreds of feet per second, do not normally travel as far as primary fragments. Thermal Hazards Generally, thermal hazards from explosives events are of less concern than blast and fragment hazards. With the release of energy from an explosion is heat. The amount of heat varies with the energetic compound (explosive). All explosives compound molecules are potentially unstable held together with weak bonds in their outer shell. When this weak bond is broken heat and energy is violently released. It normally takes longer for the thermal blast to incur. Injury from thermal effects follows the blast and fragmentation effects which happen almost instantaneously. This does not imply that there is a time lapse between blast and fragmentation effects of explosives; in fact it happens so fast that humans cannot notice the delay without specialized equipment. The time