Most of the literature agrees that most cloud to ground lightning is downward negative (~90%). Even most of lightning illustrations, depict the Earth as positive and lightning as negative Thus, it can be inferred that the Earth's surface is generally charged positively and that downward positive lightning exist only as special cases.
However, in A.Rakov and A.Uman book "Lightning - Physics and Effects" (2003), it was stated that the Earth's surface is charged negatively.
Page 17: "1.4. The global electric circuit
Shortly after the experiment at Marly that confirmed
Franklin’s conjecture regarding the electrical nature of thunderstorms
(Section 1.1), Lemonnier (1752) discovered atmospheric
electrical effects in fairweather. Further research
established that the Earth’s surface is charged negatively and
the air is charged positively, the associated vertical electric
field in fair weather being about 100 V m−1 near the Earth’s
surface." (A.Rakov & A. Uman, 2003)
Can someone please lift my confusion? Thank you in advance!
Hi Houssam, the Earth's surface is overall negatively charged, with the lower ionosphere (~70 km altitude) being approximately +300KV with respect to the ground. According to the generally accepted global electric circuit concept, this is due to a net transfer of negative charge from the world's thunderstorms and shower clouds to the surface. The charge is transferred to the surface by various mechanisms, with corona (point) discharge likely to be the largest, followed by cloud-to-ground lightning.
The lower portion of a Cumulonimbus is typically charged negatively as well. Directly under this cloud, the negative charge is of sufficient magnitude to reverse the polarity of the electric field and induce a positive charge on the conductive surface below. This is why the illustrations of the typical charge distribution below a thunderstorm rightly depict a positive surface and negative cloud base, with resultant negative cloud-to-ground lightning. Several kilometres away from the storm, the influence of the cloud charge diminishes and the polarity of the surface returns to it's "fair weather" negative value, generated by the global circuit.
Hallo Mr. Bennett,
Thank you for your clear answer. It all makes sense now.
Your help is much appreciated!
Houssam Haddad
I am unable to make proposals and appreciate the explanation given by Alec Bennett.
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