Environmental Engineering Reference
In-Depth Information
processes restrict the subsequent increase of temperature. Usually, the maximum
temperature of the lightning channel is about 3
10
4
K.
The recurrent stroke creates a hot lightning channel. Its supersonic expansion
creates an acoustic wave - thunder. Equilibrium between the conductive channel
and the surrounding air is established, and then the principal part of the atmo-
spheric charge is carried through the channel. The lightning current in this stage
of the process falls drastically in time, and the total duration of the recurrent stroke
is about 10
3
s. Then the conductive channel disintegrates. If during its decay a
redistribution of charges in the cloud has occurred, then the lightning flash can re-
cur through this channel. This can take place if the time interval from the previous
flash is no more than about 0.1 s. Then the new flash begins from the so-called ar-
rowlike leader, which is similar to the stepwise leader, but is distinguished from it
in that it travels the distance along the existing channel continuously, without delay
at each step as in the case of the stepwise leader. After the passage of the arrow-
like leader, a recurrent stroke occurs as in the first breakdown. After some delay,
breakdown may again occur by passage of an electric current through the existing
channel. As a rule, one lightning stroke contains several charge pulses through the
same channel.
Lightning is of interest as an intense source of radiation. To find the maximum
efficacy of the transformation of the electric energy into radiation, we shall assume
that the conductive channel is a blackbody of temperature 30 000 K. Then the radia-
tion flux of the conductive channel is
j
D
σ
T
4
10
5
W/cm
2
, and the radiation
5
Rj
10
7
W/cm, where the channel ra-
flux per unit length of the channel is 2
π
dius is taken to be
R
D
10 cm. Since the above temperature is maintained for
10
4
s, we have an estimated upper limit for the specific radiation energy
τ
of
ε
10
3
J/cm. The electric energy per unit length of the lightning channel is
QE
2 kJ/cm. This rough estimate leads to the conclusion that the electric energy
of lightning is transformed very efficiently into radiated energy.
Another distinguishing feature of radiation emitted by the lightning channel re-
lates to the role of UV radiation. According to Wien's law, the maximum radiation
energy for a blackbody with a temperature of 30 000 K occurs at wavelengths of
about 100 nm. This estimate shows the possibility for lightning to emit hard UV
radiation, although in reality, owing to the limited transparency of an atmospher-
ic plasma of the above temperature for UV and VUV radiation, visible radiation
constitutes the principal portion of the radiation due to lightning.
D
6.5.3
Electric Processes in Clouds
We thus consider the basis of the electric machine of the Earth to be the result
of charge separation in clouds due to the falling of negatively charged water, and
below we analyze the reality of this scheme. Then we consider the action of the
Earth's electric machine as a secondary process of water circulation in the Earth's
atmosphere. Indeed, every year 4
10
14
t (metric tons) of evaporated water pass
through the atmosphere, corresponding to 13 million tonnes of water per second,
