Electromagnetic pulse (EMP) is an electromagnetic wave similar to radio waves, which results from secondary reactions occurring when the nuclear gamma radiation is absorbed in the air or ground. It differs from the usual radio waves in two important ways. First, it creates much higher electric field strengths. Whereas a radio signal might produce a thousandth of a volt or less in a receiving antenna, an EMP pulse might produce thousands of volts. Secondly, it is a single pulse of energy that disappears completely in a small fraction of a second. In this sense, it is rather similar to the electrical signal from lightning, but the rise in voltage is typically a hundred times faster. This means that most equipment designed to protect electrical facilities from lightning works too slowly to be effective against EMP.
There is no evidence that EMP is a physical threat to humans. However, electrical or electronic systems, particularly those connected to long wires such as power lines or antennas, can undergo damage. There could be actual physical damage to an electrical component or a temporary disruption of operation.
An attacker might detonate a few weapons at high altitudes in an effort to destroy or damage the communications and electric power systems. It can be expected that EMP would cause massive disruption for an indeterminable period, and would cause huge economic damages.
On July 8, 1962, the EMP from the high altitude (250 miles above Johnston Island) "Starfish Prime" test (1.4 Mt) turned off 300 streetlights in Oahu, Hawaii (740 miles away).