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Updated : 16/10/2016

Lightning

Introduction

Living on the South African Highveld we experience a lot of lightning, in fact, we experience a ground flash density of 7.5 (strikes per km2 per year). This is bad enough until you realise that the majority of these flashes occur during the 3-4 months of summer rainfall giving a ground flash density of closer to 30 for the summer. Therefore, we have a vested interest in lightning detection and monitoring.

About Lightning

As the ice particles within a cloud (called hydrometeors) grow and interact, they collide, fracture and break apart. It is thought that the smaller particles tend to acquire positive charge, while the larger particles acquire more negative charge. These particles tend to separate under the influences of updrafts and gravity until the upper portion of the cloud acquires a net positive charge and the lower portion of the cloud becomes negatively charged. This separation of charge produces enormous electrical potential both within the cloud and between the cloud and ground. This can amount to millions of volts, and eventually the electrical resistance in the air breaks down and a flash begins. Lightning, then, is an electrical discharge between positive and negative regions of a thunderstorm.

A lightning flash is composed of a series of strokes with an average of about four. The length and duration of each lightning stroke vary, but typically average about 30 microseconds. (The average peak power per stroke is about 1012 watts.)

Thunder is the result of sound generated along the length of the lightning channel as the atmosphere is heated by the electrical discharge to the order of 20,000 °C. This compresses the surrounding clear air producing a shock wave, which then decays to an acoustic wave as it propagates away from the lightning channel.

Types of lightning

Cloud-to-ground lightning

This is the most damaging and dangerous form of lightning although not the most common type. Most flashes originate near the lower-negative charge center and deliver negative charge to Earth. However, an appreciable minority of flashes carry positive charge to Earth. These positive flashes often occur during the dissipating stage of a thunderstorm's life. Positive flashes are also more common as a percentage of total ground strikes during the winter months.

Intra-cloud lightning

This is the most common type of discharge. This occurs between oppositely charged centers within the same cloud. Usually the process takes place within the cloud and looks from the outside of the cloud like a diffuse brightening which flickers. However, the flash may exit the boundary of the cloud and a bright channel, similar to a cloud-to-ground flash, can be visible for many miles. The ratio of cloud-to-ground and intra-cloud lightning can vary significantly from storm to storm.

Inter-cloud lightning

As the name implies, occurs between charge centers in two different clouds with the discharge bridging a gap of clear air between them.

Other types

There are numerous names and descriptions of various types and forms of lightning. Some identify subcategories, and others may arise from optical illusions, appearances, or myths. Some popular terms include: ball lightning, heat lightning, bead lightning, sheet lightning, silent lightning, black lightning, ribbon lightning, colored lightning, tubular lightning, meandering lightning, cloud-to-air lightning, stratospheric lightning, red sprites, blue jets, and elves.

Reporting

Lightning is generally reported as a discharge rate e.g. discharges per minute at the measurement location. Other units are the number of discharges per unit area over a measurement period e.g. discharges per km2 per year. Professional systems are able to differentiate and report the different types of lightning.

Sensors

Available technologies of present-day lightning detectors include:

  1. Radio Frequency (RF) Detectors. These measure energy discharges from lightning. They can determine the approximate distance and direction of the threat.
  2. Inferometers. These are multi-station devices, much more costly than RF detectors. They measure lightning strike data more precisely. Usually they require a skilled operator and are employed for research purposes.
  3. Electric Field Mills (EFMs).These pre-lightning pieces of equipment measure the potential gradient (voltage) changes of the earth's electric field and report changes as thresholds build to lightning breakdown values, in the range of 15 KV. Expect only a narrow (about 5-8 km) reporting range for EFMs.
  4. Optical Monitors.These can provide earlier warning as they detect cloud-to-cloud lightning that typically precedes cloud-to-ground lightning.
  5. Hybrid Designs.These monitors use a combination of the other single-technology designs, such as RF, or light or magnetic coincidental recognition.  Two or more sources of information may be better than just one.

Many of these sensors are beyond the means of many hobbyist but there are some inexpensive yet effective radio frequency detector options available such as the one shown from HobbyBoards. In these devices the signal from an antenna, which may or may not be tuned to a specific frequency, is amplified via a darlington pair and, if above a threshold voltage, triggers a DS2423 counter via an optical isolator. The schematic of such a circuit is shown below.

I have added a 3v battery to the DS2423 in order to retain the count during any system downtime. I have also used an alternative detector circuit which uses a superior RF section with a single resonance near 300kHz.This circuit gives a lot of sensitivity but can pick up false signals form nearby electric fences and fluorescent tube starts from more than 20m away. The provision of an effective earth sytem also enhances the sensivity of the system.

Installation

Installation is fairly straight forward since the location is not critical as the system works almost as well indoors. I elected to install the detector inside the small mast I used for the anemometer system.

Sources

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