A stroke of lightning is an almost instantaneous occurrence, one which lasts for a mere hundredth of a second. Indeed Juliet, you may remember, bewitched but not befuddled by the callow youth mooning near her balcony, found it a useful simile with which to berate the hapless Romeo on the brevity of his midnight trysts: It is too rash, too unadvised, too sudden, Too like the lightning, which doth cease to be 'Ere one can say `It lightens'.
But yet, in another sense, lightning is continuous. At any given moment some 2,000 thunderstorms are in progress over the Earth's surface, and lightning strikes the ground about 100 times each second, each stroke doing its bit to neutralise the polarised electrical charges in the atmosphere. Forty-five thousand thunderstorms occur every day, or 16 million annually around the world, and scientists have found in them a useful tool to monitor the average global temperature.
Heat is the ultimate cause of every thunderstorm. The warmer the surface of the Earth, the greater tendency there is for the air to bubble upwards through the atmosphere and to initiate the towering cumulonimbus clouds from which the lightning comes. Because of this, it is in the warmer regions of the Earth that thunderstorms, and therefore lightning, are most common. Moreover, the higher the temperature, the greater the number of discharges that occur in any given period. So if we monitor the frequency of lightning flashes we can then estimate changes in the monthly average temperature of the tropical regions of the world.
Lightning flashes, however, are difficult to count directly, but scientists have discovered a way of monitoring their frequency by means of a phenomenon called "Schumann resonance". A lightning discharge produces a burst of electromagnetic radiation which is subsequently trapped between the surface of the Earth and a layer of charged particles in the upper atmosphere called the ionosphere. . Thus contained, the electromagnetic waves resonate as they surge around the globe within their narrow confines - rather like sound waves in an echo chamber.
The overall variation in this electromagnetic activity - the so-called Schumann resonance - can be measured fairly simply at a single point on the Earth's surface. Its strength is augmented as the number of lightning flashes increases, and the frequency of lightning flashes, in turn, depends on the average surface temperature in the lower latitudes. The magnitude of the resonance, therefore, is an indirect measure of the average temperature in the tropics, and it turns out to be a surprisingly sensitive thermometer, because a relatively small increase in local temperature produces a large increase in thunderstorm activity.