Environmental Engineering Reference
In-Depth Information
in the atmosphere. The action of lightning and the descent of charged raindrops
to the Earth's surface leads to charging of the Earth. The average charge carried by
a single lightning stroke is about 25 C. If we assume that all charging of the Earth
is accomplished by lightning, we conclude that it is necessary to have about 70
lightning strokes per second (i.e., about six million lightning strokes per day) for
maintenance of the observed charging current. If we assume each lightning event
is observable at a distance up to 10 km, then one can observe an average of three or
four lightning strokes every day. In actuality, the frequency depends on the season
and geographical location, but the above estimate shows that this phenomenon is
widespread.
We shall examine the distinguishing features of lightning as an electric dis-
charge. The charge separation in a cloud results from the charging of drops
(aerosols) that subsequently descend. This leads to separation of charge in a cloud,
and creates an electric potential in the lower part of the cloud that amounts to
hundreds of millions of Volt. The subsequent discharging of the cloud in the form
of a lightning stroke leads to the transfer of this charge to the Earth's surface. The
breakdown electric field strength of dry air is about 25 kV/cm, which exceeds by an
order of magnitude the average electric field strength between the Earth's surface
and a cloud during a thunderstorm. Hence, lightning as a gas breakdown has a
streamer nature. Random inhomogeneities of the atmosphere, dust, aerosols, and
various admixtures decrease the breakdown voltage. The first stage of lightning
is creation of the discharge channel. This stage is called the stepwise leader. The
stepwise leader is a weakly luminous breakdown that propagates along a broken
line with a segment length of tens of meters. A typical propagation velocity of the
stepwise leader is of the order of 10
7
cm/s, and that, in turn, is of the order of the
electron drift velocity in air in the fields under study.
After creation of the conductive channel, the electric current begins to flow
through it, and its luminosity increases sharply. This stage is called the recurrent
stroke and is characterized by a propagation velocity of up to 5
10
9
cm/s, cor-
responding to the velocity of propagation of the electric field front in conductors.
The recurrent stroke is relatively short. Its first phase (the peak-current phase)
lasts some microseconds and the discharge is complete in less than 10
3
s. During
this time the current channel does not expand, so the energy released is used to
heat the channel and for ionization of the air in it. To estimate the temperature of
the channel, we take the charge transferred to be
q
2C, withanelectric field
strength
E
in the channel of 1 kV/cm. The energy released per unit length of the
channel is
qE
, and a typical increase of the air temperature in the channel is
D
qE
c
p
Δ
T
S
,
(6.103)
10
3
g/cm
3
where
c
p
1 J/g is the heat capacity of air,
is the air density,
100 cm
2
is the channel cross section (its radius is approximately 10 cm).
From this it follows that
and
S
10
4
K. This rough estimate makes clear that the
air in the lightning channel is highly ionized. At such air temperatures, radiative
Δ
T
2
