Thunderstorm building up process.

A thunderstorm is a natural meteorological phenomenon that diplays:

-          Violent electrical discharges

-          Bright flash

-          The roll of thunder

It usually comes with strong wind, torrential rain or hail, sometimes snow.

Certain atmospheric conditions, such as high temperature or humidity, lead to storm clouds being formed. These huge, anvil-shaped cloud masses are usually of the cumulonimbus variety, the lower part being made up of water droplets while at higher altitude are found ice crystals.

Strong up currents within this type of cloud cause the electric charge on the water droplets to be separated resulting in high levels of positive charge at the top and high levels of negative charge at the bottom of the cloud.

Occasionally, a pocket of positively charged electrons is trapped low in the cloud within an area of negative charge. A storm cloud forms overhead creating a vast dipole with the ground and, under the influence of the negatively charged cloud base, the ever present electric field in the atmosphere at ground level suddenly inverts and builds up rapidly reaching between 10 and 15 kilovolts per metre. An electrical discharge to the ground is then imminent.

The first stage of a lightning strike involves an initial discharge of low luminosity known as a downward leader. It forms at the cloud centre and moves down toward the ground in steps of several dozen meters at a time (Fig.1). At the same time, the electric charge in the atmosphere at ground level increases as the downward leader gets closer. Any high point in the vicinity such as an electricity pylon or a lightning rod immediately gives rise to natural ionisation in the form of a series of electrical discharges which are blue in color. This is the point effect or corona effect.

As soon as the downward leader is close enough to the ground, the ionisation due to the corona effect intensifies, especially near any high point, and eventually turns into an upward discharge : this discharge is the upward leader that develops toward the cloud (Fig.2).

When one of these upward leaders comes into contact with the downward leader, a conductive path is created allowing a powerful current to flow. This is lightning and is characterized by its bright flash and the deafening sound of thunder (Fig.3). The lightning strike may in fact be made up of a number of successive return strokes, only a few hundredths of a second apart, all following the same highly ionised path.

Lightning hazards

Direct lightning discharge

The lightning discharge is directly hitting a person. The current then flows down the body, from the most prominent part (usually the head) to the feet and then disperses into the ground.

Lightning spark over

The lightning current flows down a low conductivity path then finds an alternative and more conductive routing through a human body nearby.

Step potentional lightning over voltage

Once the lightning current disperse into the ground, the earth electrical potential increases and generates a high voltage current in an animal or person body.

Contact overvoltage

If a person is simultaneously in contact with two conductive parts when an overvoltage occurs due to a lightning discharge, the potential difference between the two conductors may create a path for the current through the human body.

Inductive lightning

The person is struck by a secondary branch of the cloud to ground lightning discharge. Usually these branches are disappearing before ground.