Typically, when air is displaced by a pressure wave, it moves at the speed of
sound. These pressure waves can be generated from
one's vocal cords (when speaking), the vibration of an object, or the impact of
one object against another. However, in certain cases, the pressure wave is generated by a collision between air parcels, such as when lightning heats the air immediately around it. This rapidly-expanding air crashes into the cooler air around it, generating a
massive spike in pressure followed by a sharp drop-off. It is this sharp
spike and drop-off combination that differentiates a sound wave and a shock wave. Shock waves can be either naturally generated, in the case of volcanic eruptions, lightning strikes, and asteroid impacts; however, they can also be generated through man-made means, such as explosives that derive their
power either from chemical energy that is converted into thermal energy or by way of nuclear fission or fusion. If the pressure differential occurs in air where clouds can condense, the act of high pressure passing through the area and low pressure following can cause temporary "
Wilson clouds" to form.
