Lightning location

Lightning is the passage of a large spark of electricity from cloud to ground, or from cloud to cloud. The electrical charge is generated by violent vertical motions of air in storm clouds, which can also produce heavy showers of rain or hail, and squally winds. Further information about thunderstorms can be found in the leaflet on thunderstorms. A storm cloud can produce many flashes of lightning during its lifetime of several hours. Lightning is most common in equatorial regions and over land because the storms are often triggered by heating over warm ground.

The accurate location of lightning is important to public safety. As well as the obvious dangers of the lightning strike itself, thunderstorms can result in intense precipitation, severe icing, wind shear, turbulence and gusting winds. These all offer areas of concern to aviation, the construction industry, public utilities and defence.

The Met Office's Arrival Time Difference (ATD) lightning location system provides lightning location data 24 hours a day, seven days a week. The information the system provides can be used to reduce the effects of thunderstorms and lightning on human activity.

A lightning flash emits radio waves which spread out like the bands of circular ripples from a stone dropped into a pond. These radio waves travel at the speed of light. The radio waves from nearby flashes can be heard on a radio receiver as individual loud crackles, over a large range of radio frequencies. With a sensitive recorder, set at a particular frequency, there is continuous background crackling from the many distant lightning flashes that are occurring worldwide at any moment. The shape (or sound) of the burst of radio waves is unique to a particular flash on almost all occasions. Simply listening to the radio waves from a lightning flash gives no indication of where the flash occurred. However, observations of the time difference between the waves reaching two or more receivers can locate the flash with considerable accuracy.

The ATD system uses observations of the time of arrival of radio waves from flashes at a frequency of about 10 kHz. These waves travel long distances around the Earth with little loss of strength (apart from the unavoidable reduction with distance as the waves spread out) or change in shape of the wave pattern. In principle, lightning flashes can be detected from the other side of the world, but in practice the ATD system is used to locate flashes only within a range of about 8,000 to 10,000 km from the UK.

A cork floating on a pond acts as a detector of passing ripples by bobbing up and down. Several corks can detect the same ripple pattern produced by a stone being dropped into the water. However, they will detect the ripples at different times because they are at different distances from the point of impact. The ATD system works on a similar theory.

The system consists of seven unmanned, automatic lightning sensors at different locations. There are two in the UK (at Camborne in Cornwall and Lerwick in the Shetland Islands) and five located overseas, at Keflavik (Iceland), Korppoo (Finland), Norderney (Germany), Gibraltar and Cyprus.

All the outstations are linked to the control station computer located at Met Office headquarters. This automatically controls the system and collates the lightning location data into various messages for onward transmission to customers. It is also possible to reconfigure the outstations from the control station in order to optimise their performance and rectify faults. This helps make the system resilient.

The sensors at all the outstations continuously detect the radio waves generated by flashes of lightning - these are called 'atmospherics', or 'sferics'. A designated station in the ATD network then acts as the 'selector' station. The sferics received at this station are individually selected and then any sferics observed around the same time at other outstations are requested by, and forwarded to, the control station. Because each sferic has a unique waveform shape - its own 'fingerprint', which will be similar at all the outstations that receive it - the control station is able to match up the sferic waveforms received at the outstations with the particular sferic it has selected (a process operationally unique to the ATD system known as waveform correlation).

The control station then designates one outstation as the reference station for this flash. This reference station is assigned an arrival time difference of zero. The time of arrival of this particular sferic at the other outstation that received it, in relation to the reference station, is calculated. Then, by calculating all the points where the arrival time difference between the reference station and the other station are the same, a line can be plotted representing all the theoretical places with the same arrival time difference between the two stations. Drawn on the Earth's surface, this line will represent a hyperbola.

If this same process is then repeated between the reference station and another station that received the same sferic, another hyperbola can be drawn, intersecting the first one. Repeat this process using all the stations that received the sferic waveform and the control station computer will be able to determine the flash location. If the sferic was received at four or more stations then an unambiguous source location can be defined. This will be the place where all the hyperbolae intersect.

Calculated ATD hyperbolae. The point of intersection is where the sferic will be located.

The speed of the control station computer enables lightning locations to be determined in this way for anything up to thousands of flashes per hour.

A quality-control module in the control station carries out various checks on the calculated lightning flash locations ('fixes') before they are accepted. If the fix does not pass the various checks, it is rejected. In this way, whilst false fixes are not completely unknown, their rate is maintained at a very low level, ensuring confidence in the ATD system output as a whole.

The ATD system measures location accuracy in points of degree latitude and longitude. Converting from this gives the approximate location accuracies as follows:

If two lightning sensor receivers are about 300 km apart and a lightning flash occurs somewhere along the line joining the two receivers together then the arrival time difference of the sferic waveforms ranges from zero to only one millisecond (1/1,000th of a second). To obtain the required accuracy of a flash location, the time differences at two widely separated receivers must be measured to an accuracy of about one microsecond (1/1,000,000th of a second).

To achieve this each receiver has a rubidium oscillator - a form of accurate atomic clock - and the time of arrival of the sferic waveform is measured with that clock. Synchronisation of all the outstation 'clocks' is achieved by comparing them with GPS satellite time signals every ten minutes. Any drift in timing can then be identified and corrected. This ensures accurate lightning location data.