"The sudden liberation of energy causes a considerable increase of temperature and pressure, so that all the materials present are converted into hot, compressed gases. Since these gases are at very high temperatures and pressures, they expand rapidly and thus initiate a pressure wave, called a 'shock wave,' in the surrounding medium - air, water, or earth. The characteristic of a shock wave is that there is (ideally) a sudden increase of pressure at the front, with a gradual decrease behind it".... (Glasstone, The Effects of Nuclear Weapons (3rd Ed.), 1977, p. 1)