The atmosphere surrounds the earth like a blanket. The atmosphere is a system with lots of variations. Therefore only its average properties may be described. The radius of earth is about 6400 km and the atmosphere extends to about 300 km above the surface of earth which is about 1/20 th radius of earth.
When we go up, the temperature and density vary. At about 300 km, its density falls to 10-10 times its ground level value.
ELECTRICAL PROPERTIES OF ATMOSPHERE
(a) At low altitudes the atmosphere is a poor conductor. The conductivity is only due to presence of ions,
small nuclei of dirt, water vapour carrying static charge etc. The conductivity in low atmosphere varies
a lot. It even varies from day to day.
(b) At the top of stratosphere (i.e. about 50 km) the atmosphere is fairly conducting. The conductivity
increases from earth’s surface towards the top of stratosphere.
(c) At ground level there is a vertical electric field of about 100 Vm-1 all over the earth. The field weakens at higher altitudes and becomes negligible a 50 km.
(d) The potential drop from 50 km to earth’s surface is nearly 400 kV. Most of the potential drop occurs at
low altitudes.
(e) The surface density of earth is -10-9 Cm-2 the total charge being - 0.5 x 106 C.
(f) The number of protons entering the earth’s surface per sec is 2 x 107 per m2 This is equivalent to positive charge of +1800 C .
(g) To maintain the constancy of negative charge of earth and potential difference between earth and
stratosphere, there are about 4 x 104 thunder storms per day world-wise. This means a storm starts off somewhere in every two second. The duration of each storm is about 1 hour.
(h) Within each thunder cloud (or storm) positive charge is carried upward to a height of about 6 km while negative charges collect at about 2 km to 3 km above ground, the bottom of the cloud.
(i) The total amount of negative charge may be –20 C to –30 C. At the end of storm, the negative charge bursts along narrow path from cloud to earth to maintain earth at negative potential.
(j) In the last stages of a storm, there are about 200 flashes or bolts, each lasting about 2 x 10-3 sec. The peak current in each bolt is about -104 A in the downward direction. The calculation shows that each bolt deposits .20 C of charge on earth. After each bolt the thunder cloud gets charged again and gets ready for next bolt.
| NAME | ALTITUDE
RANGE IN KM | DENSITY
BEHAVIOUR | TEMERATURE
BHAVIOUR |
| Trophosphere | 0 to 12 km | falls 1 to 10-1 | Falls uniformly
from 290 K to 200 K |
| Stratosphere | 12 to 50 km | falls from 10-3 | to rises uniformly
from 200 K to 280 K |
| Mesosphere | 50 to 80 km | falls from 10-3 to 10-5 | falls uniformly
from 280 K to 180 K |
| Ionosphere | 80 to 300 km | falls from 10-5 to 10-10 | to rises uniformly
from 180 K to 700 K |
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