A thundercloud commences with the development of a cumulonimbus thunder cloud. The cloud is typically formed by rapidly raising humid air, which becomes electrified due to convection and precipitation effect. This is accompanied by wind speeds up to 200km/h.
The end result is the separation of positive and negative charges as in figure 1. in approximately 90% of the cases the lower part of the thunder cloud is comprised of a thin concentrated layer of negative charge, and the upper part of the cloud consist of more diffuse positively region. The cloud base is typically 2-6 Km.
Charge electrified due to Convention and precipitation effect. As a result of the cloud electrification a quasi electric field is established between the cloud and ground. Pointed ground objects subjected to this ambient electric field emit varying amounts of point discharge or corona, and the resulting positive or negative ions drifts upwards to form a low density “ space charge” which extends from ground to cloud. This space charge reduces the electric field observed at ground level, typically from 50-60KV/m at heights of 500m, to 2-15 KV/m at the ground.
Within confines of ground to cloud, static electricity builds to an extent where on or more neutralizing discharge or flashes occur. These flashes can be in the form of an inter cloud or cloud to ground flashes.
The dramatic cloud to ground is of most concern. The dynamitic phase of lighting commences in the form of a luminescent downward leader from the base of the cloud, which proceeds in a series of steps and branches towards the ground. The protrusion of ground objects into a ambient field at the tip of the object. As the down leader approaches, it cause the electric field around points on the surface of the earth to increase rapidly, leading to the initiation of small upward streamers from the elevated points. Under the right conditions, the upward streamers thermalise and become competing upward leaders, which propagate toward the approaching down leader.
The ability of one ground point to develop an upward intercepting leader before other nearby competing points that can become the preferred strike point to successfully complete an ionized path between cloud and ground.
A small proportion of flashes transfer positive charge to ground (positive lightning). Typically 10% of lightning flashes are positive, although this can vary with latitude and season. The parameters for positive lightning differ considerably from their negative counterparts. Some of the main differences are that the:
· Strike phenomenon is absent (no subsequent strokes)
· Peak current higher ( ~ 2 x)
· Maximum rate of rise of current is less ( ~ 0.1 x)
· Total rise time is longer ( ~ 4 x)
· Stroke duration is longer ( ~ 4 x)
· Action integral (energy content) is higher ( ~ 10 x)
In summary, the main lightning discharge is characterized by a rapidly rising current (averaging about 30,000 Amps) with maximum values exceeding 200,000 Amps. This whole process is extremely rapid, typically occurring within milliseconds. The average energy released in a single discharge may be 55 kW hours. The danger lies in the extremely high rate of current rise (up to 1010 Amps per second) which can generate very high voltages, and also from the continuing current following the peak.
Without a proper intervention to capture and control the passage of this lightning energy to ground, cloud-to-ground lightning can be catastrophic.
The end result is the separation of positive and negative charges as in figure 1. in approximately 90% of the cases the lower part of the thunder cloud is comprised of a thin concentrated layer of negative charge, and the upper part of the cloud consist of more diffuse positively region. The cloud base is typically 2-6 Km.
Charge electrified due to Convention and precipitation effect. As a result of the cloud electrification a quasi electric field is established between the cloud and ground. Pointed ground objects subjected to this ambient electric field emit varying amounts of point discharge or corona, and the resulting positive or negative ions drifts upwards to form a low density “ space charge” which extends from ground to cloud. This space charge reduces the electric field observed at ground level, typically from 50-60KV/m at heights of 500m, to 2-15 KV/m at the ground.
Within confines of ground to cloud, static electricity builds to an extent where on or more neutralizing discharge or flashes occur. These flashes can be in the form of an inter cloud or cloud to ground flashes.
The dramatic cloud to ground is of most concern. The dynamitic phase of lighting commences in the form of a luminescent downward leader from the base of the cloud, which proceeds in a series of steps and branches towards the ground. The protrusion of ground objects into a ambient field at the tip of the object. As the down leader approaches, it cause the electric field around points on the surface of the earth to increase rapidly, leading to the initiation of small upward streamers from the elevated points. Under the right conditions, the upward streamers thermalise and become competing upward leaders, which propagate toward the approaching down leader.
The ability of one ground point to develop an upward intercepting leader before other nearby competing points that can become the preferred strike point to successfully complete an ionized path between cloud and ground.
A small proportion of flashes transfer positive charge to ground (positive lightning). Typically 10% of lightning flashes are positive, although this can vary with latitude and season. The parameters for positive lightning differ considerably from their negative counterparts. Some of the main differences are that the:
· Strike phenomenon is absent (no subsequent strokes)
· Peak current higher ( ~ 2 x)
· Maximum rate of rise of current is less ( ~ 0.1 x)
· Total rise time is longer ( ~ 4 x)
· Stroke duration is longer ( ~ 4 x)
· Action integral (energy content) is higher ( ~ 10 x)
In summary, the main lightning discharge is characterized by a rapidly rising current (averaging about 30,000 Amps) with maximum values exceeding 200,000 Amps. This whole process is extremely rapid, typically occurring within milliseconds. The average energy released in a single discharge may be 55 kW hours. The danger lies in the extremely high rate of current rise (up to 1010 Amps per second) which can generate very high voltages, and also from the continuing current following the peak.
Without a proper intervention to capture and control the passage of this lightning energy to ground, cloud-to-ground lightning can be catastrophic.
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