Sound and Fury
Image courtesy of Steve Albers at NOAA
On clear days and nights there is a slow, steady, nearly uniform flow of electricity from the surface of the negatively charged earth and oceans to about 50 km up into the positively charged atmosphere. The net charge over the whole earth is about 10^6 C; the potential difference between the earth and the electrosphere is about 3.0E5 Volts; and the current density is about 1.0E-11 amps/m^2. Where there are no clouds, between the earth and the 50 kilometer elevation, the electric field may reach values as high as 100 volts per meter. In thunderclouds the electric field may be much higher. Lightning is the means by which electrons are transported back to the earth to "recharge" the earth to its negative polarity.
Solar energy is the ultimate source for creating lightning. Warmth from the sun's radiant energy is responsible for convection of air, snow, rain, and hail within clouds. Through frictional effects (very much like those which make static electricity when we walk across carpets) the flow of air masses and precipitating droplets cause separation of charges. Free charges created in the upper atmosphere by cosmic rays also play a role in the detailed behavior of lightning.
The electrical structure in thunderheads (generally cumulonimbus clouds) is complex. There are major differences between summer and winter thunderheads, and between those over Florida, New Mexico, South America and Japan. For the most common type of lightning, negative cloud to ground, there is a strong negative region in the lower section of the cloud. The base and top of the cloud may also have scattered positive regions.
The lightning flash is composed of several strokes. The first, called the stepped leader, originates from the cloud. It comprises brief (~ 1 ms) spikes in electrical current (to more than 1000 A) separated by times of lesser current ( 50 ms, ~100 A). This part of the lightning flash sets the jagged shape that the later, more intense return strokes (currents originating from the earth) will follow. The return strokes may have currents in excess of 30 kA, each lasting for about 50 ms. A typical lightning flash transfers about 10 C from the atmosphere to the earth.
|Voltage (relative to ground)||1.0E+8 to 1.0E+9 V|
|Peak current||30,000 A|
|Duration||0.001 - 0.5 s (bursts separated by times of relative calm)|
|Diameter of current channel||0.1 m|
|Diameter of luminous region||1-10 m|
|Typical Length||5.0E3 m|
|Plasma Temperature||to 3 eV|
|Electron Density||1.0E+23 to 1.0E+24 electrons/cubic meter|
|% power into visible light||1-3|
|% power into sound||10-50|
|% power into heat||10-50|
|% power into radio waves||10-50|
|Peak magnetic field created||1000 G|
Page Written by Sam Cohen and Hannah Cohen.
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