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. (Fig.1)
fig .1 : Electric charges distribution in the cloud and ground electric field value.
The build up to a lightning strike
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.2a). 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 colour. This is the point effect or corona effect as observed by sailors during a storm, known as Saint Elmo’s fire, or by mountaineers who report hearing the characteristic humming of «bees» prior to a storm. 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.2b)
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 characterised by its bright flash and the deafening sound of thunder. (Fig.2c) 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.
fig.2 : Les phases caractéristiques de l’évolution d’un coup de foudre négatif.
Different types of lightning
In temperate climates like ours, the vast majority (about 90%) of lightning is of the negative downward variety where the discharge is from the negatively charged cloud base down to the ground. (Fig.3a)
Occasionally - usually during the winter - a downward leader may build up within a positively charged pocket in the base of the cloud and discharge down to the ground. This type of lightning is known as positive downward.(Fig.3b)
When electrical conditions in the atmosphere permit, an upward leader variety of lightning may shoot up spontaneously from a point of high altitude (e.g. a mountain peak, telecommunications tower or high building).
The resulting lightning is known as positive upward (Fig.3c), or, less frequently, negative upward (Fig.3d), lightning depending upon the cloud’s electrical charge.
Types of lightning
negative downward lightning
Cachoeira Paulista (Brazil)
Positive upward lightning Nadachi