Download Oppgavesett kap. 6 (4 av ..) GEF2200

Oppgavesett kap. 6 (4 av ..)
GEF2200
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Oppgave 1
(a) Where do we find the main charging zone?
• We find it rather well defined between -10 and -20◦ C. This is the location of
the negative charge.
(b) Where do we get the most thunder, in warm clouds or cold clouds?
• There are more thunderstorms in cold clouds.
(c) We know from observations that electrification of a cloud follow after heavy
precipitation. Explain how this can lead to charging. Relate this to what
you know about riming.
• When hail or graupel falls through the cloud, they will collide with cloud
droplets (liquid or ice) and most theories assume that these collisions can result in the transfer of negative charge from the cloud droplets to the hail/graupel.
Thus the negative charge region is created. One proposed mechanism by which
this could happen is as follows: Charge is transferred when ice particles collide
and rebound. The direction of the transfer is dependent on the relative growth
rate from the vapor phase of the colliding ice particles. This is appearantly
due to the fact that positive ions move faster through ice than negative ions.
As the ice particle grows by deposition, the positive ions move fastest into the
ice, away from the air-ice interface. If one ice particle grows faster than the
other, this one will have more negatively charged surface than the other and
will receive positive charge from the other. It so happens that as graupel/hail
falls through the cloud, it’s temperature will rise because of latent heat release
when supercooled droplets freeze onto it. This results in the graupel growing
slower from the vapor phase than the ice particles. Therefore, these become
negatively charged and creates the main charging zone.
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(d) Explain how the transfer of charge between two ice particles depend on their
growth rate.
• See previous question
(e) What path does a lightningbolt moving from above (ground flash) take?
Why?
• These originate in the negative charge center. A discharge, called the stepped
leader, moves downward towards the earth in discrete steps (about 50m, time
50µ s between steps). It is believed that this stepwise discharge is initiated
by a local discharge between the small pockets of positive charge at the base
of the the thundercloud and the lower part of the negatively charged region.
As the negative charge moves downward, it induces a positive charge at the
ground. When it is around 10-100 m from the ground, a discharge moves up
from the ground to meet it. Then, a large amount of electrons flow to the
ground and a highly luminous and visible lightning stroke propagates upward
from the ground to the cloud following the same path as the discrete steppes.
(f) Why should you never hide under a tree during a thunderstorm?
• Because the negatively charged stepped leader will particularly charge positively protruding objects, such as trees. The lightning will thus be more likely
to “strike” the tree.
(g) Does lightning move up or down?
• Both.
(h) What is the “fair weather electric field”?
• We know that the in a thunderstorm, the charge is separated in a negative
region (main charge region) and a positive region higher up. This charge
can then be conducted upward to the elecrosphere. This again sets up a
fair weather field where we don’t have thunder clouds. Budget for earth:
90 units of positive charge gained through the fair weather conductivity, 30
units gained from precipitation, 100 units lost through point discharges and
20 units lost due to the transfer of negative charges to the earth by ground
lightning flashes.
(i) Explain figure 6.55.
• See (h).
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6.8 in the book
(mm) The precipitation particles are polarized by the field they are in and preferentially collect ions with opposite charge of the ground.
(pp) Because the charge separation depends on the growth of of ice particles from
the vapor phase.
(qq) Because large updrafts are needed to be able to carry larger particles upward.
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