Frequently Asked Questions

Lightning is made of several types of electrical discharges aiming at neutralizing the electrical charges distributed in the thundercloud and on the ground. These discharges, generated by the strong electrical fields present inside the cloud and between the cloud and the ground, transfer electrical currents ranging from few amperes up to several hundred.

When a discharge occurs between the cloud and the ground, regardless its direction (downward or upward), it is called a return-stroke or a stroke. Several return-strokes may flow in one or several channels, each channel creating a ground strike point. When the discharge remains inside the cloud, it is defined as an Intra-Cloud (IC) pulse.

A lightning flash is the result of the occurrence of all the discharges, strokes or IC pulses. For historical reasons, lightning locating system operators define a Cloud-to-Ground (CG) flash based on the characteristics of the first return-stroke, or of the first IC pulse for an IC flash.

Vaisala’s LS7002 sensors provide two key parameters used to localize strokes: the incoming direction of the LF (Low Frequency) electromagnetic signal in respect to the geographical North and its time of arrival at the sensor. This allows networks composed of such sensors, which is the case of the METEORAGE network, to use the Vaisala’s IMPACT method (IMProved Accuracy from Combined Technology) which combines both angle and time to accurately locate stroke positions. Combining angle and time measurements guarantees a high resolution and optimum lightning data quality with a minimum number of sensors.

Sensors from other manufacturers only provides the time of arrival because this requires simpler hardware, with one electromagnetic antenna instead of two, thus making the sensors less expensive. The consequence is that with a similar number of sensors, their network will provide a significantly lower data quality.

Two different methods are typically used by lightning detection networks to do this IC/CG discrimination:

• Waveform recognition. This is what is used by the METEORAGE network and other networks of Vaisala technology. Signals generated by CG and IC discharges are different enough to allow an efficient classification of the discharges by the TLP collecting measurements from a “normal” commercial network, meaning a baseline between sensors in the hundreds of km.
• Altitude measurement. Technically, the angles of arrival can be used to determine the altitude, and thus differentiate ICs from CGs. However, the measurement uncertainty from hundreds of km away is far too important to be a reliable IC/CG discrimination parameter. Claims that this can be done properly are misleadingly based on the case study of an extremely dense network covering a single city, where dozens of sensors were installed with a baseline of a few tens of km. Obviously, an entire country cannot be covered like this as it would require thousands of sensors.

As with every measurement, while the discrimination process between IC and CG is optimized, it isn’t perfect. So, it might still happen sometimes that a CG stroke is mistakenly characterized as an IC stroke, and vice versa.

When there is a strong suspicion of lightning-related incident but no matching CG stroke, we recommend having a look at the IC activity as a secondary safety step, to see if there isn’t an IC matching the precise time and location of the incident and which might be a mis-characterized CG.

A reliable IC/CG discrimination is also crucial to adequately compute lightning density values (Nsg/Ng), as explained in the IEC 62858 international standard.

By definition, Intra-Cloud pulses do not hit the ground and cannot cause damages. However, detecting them is useful for three main applications:

• For real-time services, such as lightning alarms and the tracking of thunderstorms, intra-cloud activity is crucial as most of the electrical activity in a thunderstorm remains in the atmosphere (about 70 to 90% of the discharges are IC pulses). A storm usually starts with intra-cloud pulses before anything hits the ground. So, a thunderstorm can be spotted sooner, and an alarm can be triggered using such events, giving our customers more time to react.

• Thunderstorm cells sometimes exhibit a sudden increase of the frequency of lightning flashes, called the Lightning Jump, which is mainly driven by IC flashes. This lightning jump is used to anticipate severe phenomena in a thunderstorm like strong winds, hail and heavy precipitations.

• Upward lightning flashes are a specific type of lightning which isn’t common but can be triggered by very tall structures such as wind turbines. They are typically composed of both CG strokes and IC pulses.