it was an early form of a heavy duty surge-protection device. Modern arresters, constructed with metal oxides, are capable of safely shunting abnormally high voltage surges to ground while preventing normal system voltages from being shorted to ground. In 1962, the USAF placed protective lightning strike-diversion tower arrays at all of the Italian and Turkish Jupiter MRBM nuclear armed missiles sites after two strikes partially arming the missiles. Monitoring and warning systems The exact location of a lightning strike and when it will occur are still impossible to predict. However, products and systems have been designed of varying complexities to alert people as the probability of a strike increases above a set level determined by a risk assessment for the location's conditions and circumstances. One significant improvement has been in the area of detection of flashes through both ground- and satellite-based observation devices. The strikes and atmospheric flashes are not predicted, but the level of detail recorded by these technologies has vastly improved in the past 20 years. Although commonly associated with thunderstorms at close range, lightning strikes can occur on a day that seems devoid of clouds. This occurrence is known as "a bolt from the blue [sky]"; lightning can strike up to 10 miles from a cloud. Lightning interferes with amplitude modulation (AM) radio signals much more than frequency modulation (FM) signals, providing an easy way to gauge local lightning strike intensity. To do so, one should tune a standard AM medium wave receiver to a frequency with no transmitting stations, and listen for crackles among the static. Stronger or nearby lightning strikes will also cause cracking if the receiver is tuned to a station. As lower frequencies propagate further along the ground than higher ones, the lower medium wave (MW) band frequencies (in the 500–600 kHz range) can detect lightning strikes at longer distances; if the longwave band (153–279 kHz) is available, using it can increase this range even further. Lightning-detection systems have been developed and may be deployed in locations where lightning strikes present special risks, such as public parks. Such systems are designed to detect the conditions which are believed to favor lightning strikes and provide a warning to those in the vicinity to allow them to take appropriate cover. Personal safety The U.S. National Lightning Safety Institute advises American citizens to have a plan for their safety when a thunderstorm occurs and to commence