Storm Indicator

an instrument for recording storms within a given radius, called the effective radius of the storm indicator. The storm indicator’s operation is based on detecting lightning within the indicator’s effective radius.

Depending on the type of signal received, indicators are classified as acoustical, optical, or electrical; they record thunder, flashes of light, or changes in electromagnetic field that accompany lightning, respectively. Electrical storm indicators are the most widespread and include indicators that register magnetic fields caused by the passage of the lightning’s current, indicators that respond to the electrostatic field changes that occur when the charges of clouds are neutralized and displaced during lightning flashes, and indicators that receive electromagnetic radiation—radio waves—when the current of lightning changes.

The world’s first radio receiver, created by A. S. Popov (1895). was the first storm indicator to receive electromagnetic radiation from lightning. Storm indicators of this type can have an effective radius in excess of thousands of kilometers, whereas other types are not effective over a radius of more than 100 km. Since the parameters on which the operation of storm indicators is based can vary for different types of lightning by hundreds or thousands of times, the effective radius of operation of a storm indicator is a statistical quantity determined by prolonged observation.

The main elements of any electrical storm indicator are an antenna to receive a signal from lightning; a relay device triggered by the incoming signal; and an indicator or register that marks the occurrence of the signal. A storm indicator equipped with a recording device is called a storm recorder. The latter can also serve to count the number of lightning flashes that occur within the effective radius of the storm indicator over a rather long period of time. In this case, the storm indicator is called a lightning counter.